There are times when various words can both be used to describe one particular kind of animal. The animal one word specifies is a bit different from the other(s), but in most cases there's plenty of overlap and few know the little things about them that differentiates them.
crocodile and alligator
turtle and tortoise
shrimp/prawn and lobster
crab, lobster, and crayfish
EDIT: I know, the examples are a bit confusing. I wrote this in a hurry, thinking that I'll edit it later on (which I have done). The species/catagory these words fall into is the same.