I've done a few for my neighbors but with a larger backhoe. The hardest part is often moving the animal - getting it into the bucket, making sure the kids aren't around. I usually allow 4-6 hours for it: they haven't found a good spot, the animal is in an awkward place and hard to move, the ground has surprises.
Hardcore folks put all dead animals (horses, cows, sheep, deer) way out back and then later sell the skulls to local artists/ antique shops. But some folks think of horses as pets to be buried w/ dogs & cats.