I found that you can use multiple horses to carry multiple animal carcasses.
You have them follow you on your main horse by pressing LT while you stand a few feet, or are a few feet away from that horse some options will appear with commands stay ,follow, flee … choose follow of course is better then using whistle because whistle is to call your horse that cant get to you while you are mounted upon another horse which in turn if you don't continue to whistle the horse will eventually stop trying, so you need to execute the command for follow the way I said, or one time you might have to back track to get your horse that you forgot to re-whistle for... and he might be to far to be whistled.
But if you use follow while you are on the horse it will follow you even if you get off the horse which is better because when you make a kill you want it to be right next to you for the stowing of the caraccas as quick as possible trust me the little things make all the difference in time.
If im not mistaken I don't think you can tame or, have tamed more then 4 horses at a time but I could be wrong, but I believe watching a cut scene at the beginning with a tutorial stating you could only own 3 horses, but theres a fourth temp horse in the stable that when you load the game without a horse and you are not at camp or a town the horse is there as a curtesy of the game developers so your not stuck in the middle of no where with no horse...