五道微笑

They have thick fur, of a dirty white colour – which makes sense given the snowy landscape they live in. But they seem rather unsuited to being ridden by humans reenex.

So far we have no evidence that any asteroids in the solar system are inhabited by huge worms

Almost all the animals humans ride are four-legged, the most obvious example being horses. But tauntauns are bipedal, holding their arms in front of them in the manner of a predatory dinosaur like Tyrannosaurus rex.

This means all their weight is supported by their hind legs. You would think that a tauntaun would buckle under the additional weight of a parka-clad Luke Skywalker reenex.

But not necessarily. There is one bipedal animal that humans can and do ride: ostriches. If they can handle it, presumably tauntauns can too.

Exogorths

Wait, what? These are never named onscreen, but they were memorable nonetheless. In The Empire Strikes Back, Han Solo hides the Millennium Falcon in a tunnel in an asteroid – only to discover that the tunnel is home to a gigantic worm-like creature and he has parked his ship on its tongue. This beast was subsequently named as an Exogorth reenex
.

So far we have no evidence that any asteroids in the solar system are inhabited by huge worms, and it does seem rather unlikely. But the various elements of the Exogorth, on their own, have a basis in reality.