When something is rumpless, it means it doesn't have a rump, which is the back part of an animal's body.
Being rumpless is just a way to describe how something looks, like a kind of animal.
Examples:
The bird is rumpless, so it looks different from the others.
The rumpless cat is funny to look at.
I saw a rumpless dog at the park.
The rumpless chicken looks like it's missing something.
The rumpless horse is very strange to see.
The rumpless cow is a very rare animal.