If you want to describe an alien or fantastic monster to a group of players, avoid saying anything like "it's like a <real world animal>". Because that just leads to an endless stream of questions and assumptions which might not line up with what the creature is really like.

Although on the other hand, if you say "it's like a hamster" and it actually has razor sharp venomous claws and fangs, then that might be good for a few laughs.

Transcript

GM: The fleet hovers over a giant pit in the sand. Spines are arrayed to prevent easy escape.
Chewbacca: Didn't you say there were tentacles?
GM: And tentacles.
Chewbacca: And teeth?
GM: Yes, monstrous sharp teeth.
Han: And butter.
GM: Well, it's not really a snail. More like an octopus.
Chewbacca: So it'd have a beak, then?
R2-D2: Yeah. A beak full of teeth.
C-3PO: Ooh! And it has a body extending hundreds of metres below the sand, with thousands of shredding limbs!
Luke: It would be nice if you could stop making this worse.