a lion
That happened so long ago that there's no answer to be had. Since man is an animal, and Cain killed Abel (both men and animals) man may well have been the first animal killed. But like David ansered, there is no real answer that can be proven.
2,138,592 b.C.
adam
a bear
David Johnston
David killed Goliath.
It is illegal to not take a (an) animal that you have shot and killed.
David killed Goliath by hitting him with a stone thrown from a slingshot.
The crocodile has been killed by man and man is an animal
Iagoo told him to kill a famous roebuck, a deer with antlers
David killed his self by using drugs.
When david killed the bear and the lion he was a young man, and the youngest of Jesse's sons.