A Roman fresco depicting bulls.
"Bulls are selected as the very choicest of victims, and are offered up as the most approved sacrifice for appeasing the gods. Of all the animals that have long tails, this is the only one whose tail is not of proportionate length at the moment of birth; and in this animal alone it continues to grow until it reaches its heels. It is on this account, that in making choice of a calf for a victim, due care is taken that its tail reaches to the pastern joint; if it is shorter than this, the sacrifice is not deemed acceptable to the gods."
Pliny the Elder - Naturalis Historia (the Natural History) VIII 70
Translated by John Bostock, H.T. Riley