Respect for animals and nature in general is a very modern concern. There are numerous ancient texts that involve explicit reflection on the need to respect the “environment”, among other reasons because the ability to destroy or modify was much lower and this probably makes unnecessary that reflection.
But certainly there are numerous references in ancient texts that advise the man "to live in accordance with nature", but the point of view of this advice is not focused on nature conservation but in the way of life of the man himself.
Nor is there an explicit awareness of today called "animal rights"; rather, the widespread opinion is rather that the animals, which do not have souls, at least as human, they do not have "rights".
It is an exception is the belief and vital rule of Pythagoras and his disciples, and several other thinkers who believe in metempsychosis or transmigration of souls, also in animals and therefore they can not eat animal flesh and they are vegetarians.
Because all this there are some particularly interesting and shocking texts in the above context, including some laws that obliged to respect and not mistreating loading animals of "cursus publicus" or "state transport service and official communications of the Empire" .
This "transport service" is a remarkable Roman creation attributed to Augustus and developed remarkably in the Empire. There were notable precedents, like the system by changing horses in order to transmit or report messages w created by the Persian king Darius I, on the "royal road", that I referred in http://www.antiquitatem.com/en/post-service-road-mail-cursus-publicus .
On another occasion I will discuss the issue of cursus publicus at greater length and detail; it is enough today know that this "public" or state service was created, not surprisingly, on the structure of the special system of "roads" , "viae", which go to Rome, "the city par excellence”. Along the way, every twenty or thirty kilometers, there were some stations, guesthouses or hostels where rest overnight and change animal for transport of passenger or cargo .
Relatively speaking, they were something like our stations or service areas on our roads and highways.
Well, this "cursus publicus" is highly regulated by law. The Codex Theodosianus On the post, carriages and complementary services "(In angariis et parangariis)" Sixty-six regulatory provisions of service are precisely preserved on Book VIII, Title 5, that is titled “De angariis et parangariis " “The public post, post wagons and supplementary post wagons” .
Among them there are some that require treat well post and cargo animals. The most striking of these is the 8.5.2 that prevents excesses and cruelty in the treatment of these animals:
Codex Theodosianus, 8.5.2 The public post animals (cursus publicus) only should be forced by switch or whips, but not by clubs.
The same Augustus to Titianus
Since many people require at the beginning of their course the public animals to exhaust all the strength they have by knotty clubs, it is my pleasure that no person in any way shall use a club to spur them, but he shall use either a switch or a whip with a little sting tied in the tip, by which the lazy members may be gently stimulate, but not to try to get everything that forces of animals can not give.
If a person (soldier) of high gradation should act against this law, he shall be punished by humiliation of degradation; if a simple operator (soldier), he shall receive the penalty of deportation.
Give on the day before the ides of May in the consulship of Sabinus and Rufinus. May 14, 316
CTh.8.5.2 De animalibus cursus publici,virga tantum, aut flagro, non fustibus cogendis
Idem a. ad Titianum.
Quoniam plerique nodosis et validissimis fustibus inter ipsa currendi primordia animalia publica cogunt quidquid virium habent absumere, placet, ut omnino nullus in agitando fuste utatur, sed aut virga aut certe flagro, cuius in cuspide infixus brevis aculeus pigrescentes artus innocuo titillo poterit admonere, non ut exigat tantum, quantum vires valere non possunt. Qui contra hanc fecerit sanctionem promotus, regradationis humilitate plectetur: munifex poenam deportationis excipiat. Dat. prid. id. mai. Sabino et Rufino conss. (316 mai. 14).
This law is from time of Constantine. The rule punishes the excesses and cruelty in the treatment of animals of cursus publicus and punishment is depending on the category of offender: if the offender is of no gradation, it is certainly very severe.
Some others laws regulate precisely the weight and burden that the various types of vehicles may to carry and the animals themselves. You can also see in them a tinge of respect and consideration for animals.
On CTh.126.96.36.199 y 2:
We determine that the four-wheeled cart (Raeda) will only be loaded with thousand pounds, the two-wheeled cart (birota) with two hundred, a horse with thirty; it does not appear appropriate that they cannot support heavier weights.
That in summer eight mules shall be yoked to the four-wheeled cart (raeda), but ten in winter. We consider believe that three mules are sufficient for a two-wheeled carts (birota). And we ordain that the supervisors of each region shall administer all these issues, under penalty legally established for them.
Given on the eighth day before the Kalends of July at Milan, in the ninth consulship of Constantius Augustus and in the second consulship of Julian Caesar. (June 24, 357 )
Statuimus raedae mille pondo tantummodo superponi, birotae ducenta, veredo triginta; non enim ampliora onera perpeti videntur. (357  iun. 24).
Octo mulae iungantur ad raedam aestivo videlicet tempore, hiemali decem; birotis trinas sufficere iudicavimus. Adque haec cuncta regionibus praestitutos curare praecipimus poena eis proposita. Dat. VIII kal. iul. Mediolano Constantio a. VIIII et Iuliano caes. II consulibus. (357  iun. 24).
And the same weight limitation requirements are repeated on CTh.8.5.17, a norm of March 14, 364, where they are fixed sentence to exile for the free citizen and to work in the mines for slave (si liber sit, exilii poenam, si servus, metalli perpetua supplicia subeunda)
Emperors Valentinian and Valens Augustuses to Menander.
We do not shall allow to charge vehicles beyond a thousand pounds, and mail users will have sufficient the concession to carry thirty pounds on horses. All things considered that exceed this measure must be confiscated to the Treasury at the expense of the person who has broken the law.
To end completely the use of huge vehicles, we decree also quite that if any manufacturer suppose that he might build a vehicle bigger than the rule says we just prescribe he shall not doubt that if he is free, he will suffer the punishment of exile and if he is a slave, he will suffer the punishment of perpetual work in the mines.
Given on the day before the ides of March at Milan in the consulship of the divine Jovian and of Varronianus. March 14, 364
CTh.8.5.17pr. y 188.8.131.52 De mensura seu modo ponderis et vehiculorum
Impp. Valentinianus et Valens aa. ad Menandrum. Vehiculis nihil ultra mille librarum mensuram patiemur imponi, ita ut veredarii sat habeant, quod his triginta libras equis vehere concessimus. Quidquid igitur supra mensuram exsuperare constiterit, ad dispendium eius, qui in legem conmiserit, fisco conveniet adscribi.
Illud sane, ut penitus enormium vehiculorum usus intercidat, sanciendum esse decernimus, ut, quisquis opificum ultra hanc quam perscripsimus normam vehiculum crediderit esse faciendum, non ambigat sibi, si liber sit, exilii poenam, si servus, metalli perpetua supplicia subeunda. Dat. prid. id. mart. Mediolano divo Ioviano et Varroniano conss. (364 mart. [?] 14).
And again these loads and weights are specified on CTh.8.5.28 De pondere seu onere redae, angariae, veredi, where these rules applied in Gaul are extended to Italy and to Ilyrium:
On weight or load to four-wheel carriage (Raeda), to the wagons (angaria) and to post horses.
The same Augustuses to Probus, PraetorianPrefect.
Since already it is very helpful to Gaul, it should now be repeated to Illyricum and to the regions of Italy: that in the four-wheeled cart (Raeda) it may be transported not more than a thousand pounds; one thousand five hundred are sufficient for a wagon (angaria, with a team of four oxen), and no person shall impose more than thirty pounds upon a horse.
Dispatched in Sirmium on the fifth day before the calends of January in the consulship of Valentinian and Valens, Augustuses. December 28, 368.
CTh.8.5.28 De pondere seu onere redae, angariae, veredi
Iidem aa. ad Probum praefectum praetorio.
Quod iam Gallis prodest, ad Illyricum etiam Italiaeque regiones convenit redundare, ut non amplius raeda quam mille pondo subvectet, angariae mille quingenta sufficiant, veredo ultra triginta nullus imponat. Directa V kal. ian. Sirmio Valentiniano et Valente aa. conss. (368? 370? 373? dec. 28).
And once again on CTh.8.5.47. Load or weight that may to be put on vehicles and post horses.
The same Augustuses to Cynegius, Praetorian Prefect.
The four-wheeled cart shall be loaded with a weight of a thousand pounds and the car with no more than six hundred, adding that gold and the various types of products which are distributed shall be conveyed not on vehicles selected by escorts and tax collectors, but on the vehicles appropriated for the weight and burden. These of course will not be allowed under the threat of capital punishment to take any private burden cargo contradicting what our law prescribes nor to carry as a gift other persons who should be transported as if it were an auction, except for those who are required by the need of escort.
And since the care of post horses must be treated with the same rationality, the saddle and the bridle shall not pass beyond sixty pounds and certainly saddlebags beyond thirty-five, with the proviso that if any person exceeds the limits prescribed by the imperial moderation, his saddle shall be cut into pieces, and the saddlebags shall be allocated to the accounts of the treasury.
Given on the fifteenth day before the calends of July at Constantinople in the first consulship of Arcadius Augustus and in the consulship of Bauto- June 17, 385.
CTh.8.5.47pr. De onere seu pondere vehiculis et veredis imponendo.
Idem aaa. Cynegio praefecto praetorio.
Raedae mille librarum onus imponi debet, carro sescentarum nec amplius addito eo, ut aurum ceteraeque species largitionales non ad libidinem prosecutorum vel susceptorum, sed aptis oneri ac ponderi vehiculis deferantur. Quibus utique non licebit sub capitalis exitii minis quicquam oneris privati secus quam lex nostra praescribit imponere neque alios mercede subvehendos velut proposita licitatione conducere, exceptis his quos necessitas prosecutionis adiunxerit. (385 iun. 17).
Et quoniam veredorum quoque cura pari ratione tractanda est, sexaginta libras sella cum frenis, triginta quinque vero averta non transeat, ea condicione, ut, si quis praescripta moderaminis imperatorii libramenta transscenderit, eius sella in frusta caedatur, averta vero fisci viribus deputetur. Dat. XV kal. iul. Constantinopoli Arcadio a. I et Bautone conss. (385 iun. 17).
There's even a norm for ensuring the adequate feed of animals of official post.
CTh.8.5.60 On fodder of animal of official post (cursus publicus)
The same Augustuses to Messala, Praetorian Prefect.
If the forage is valued at an excessively high and unfair price, the public animals (official post) are clearly harmed by supervisors (of the service) and the escorts (apparitores) of magistrate. To prevent this happening, your "sublimity" shall provide that forage does not lack at the changing stations nor tax for the provincials shall be heavier than it which justice reasonably allows.
Given on the fifth day before the calends of December at Milan in the consulship of Stilicho and Aurelianus. November 27,400
CTh.8.5.60 De pabulo animalium cursus publici
Idem aa. Messalae praefecto praetorio.
Animalia publica, dum longe maiore ac periniquo pretio pabula aestimantur, per mancipes adque apparitores aperte vexantur. Ne id contingat, sublimitas tua disponat, ut neque pabula mutationibus desint neque provinciales ultra, quam iustitiae sinit ratio, praegraventur. Dat. V k. dec. Mediolano Stilichone et Aureliano conss. (400 nov. 27).
Someone may think that these norms are not so much by respect for the animal as a living being, similar to a man, than regarding an economic interest for not to destroy a necessary tool. Each one shall judge previous texts as he will considerer relevant.
Of course these texts contrast with the use that the Romans themselves make of the animals, if more exotic best, for all kinds of shows in the amphitheater. As they contrast also with the fun and joy that they arise in many sectors of society today the acts of atavistic cruelty like unhappy and famous "Toro de la Vega", (an Spanish bull festival) and many dozens of similar examples that the "tradition" can not justify morally.