Except everything about it and it's names in every language point to hedgehogs? While fairly closely related to the gymnures, which look very much like basal mammals (except, you know, not being monotremes), hedgehogs are pretty well differentiated.
Like most species that use rollout, it's because Chespin can roll into a ball, like the hedgehog and chestnuts he is based upon. Cyndaquil can learn rollout by level up. As can Voltorb. And Miltank. And Qwilfish. And Solosis. What they have in common is that they are either spherical or can roll up into a sphere. (Spheal DOESN'T get rollout by level up - but they do get ice ball, which is basically rollout with an ice typing) -- In fact, it's the vast minority of those that can learn rollout that are rock types. The fact it learns rollout reinforces the idea that it is a hedgehog, capable of rolling up into a spiny ball.
Rollout really does not suggest a rock typing for Chespin - I would be more shocked if it DIDN'T get rollout, based on the species it's based on.
You pose a good point with the Rollout thing. Actually, the relevance between it learning Rollout and being able to curl up into a ball seemed to have escaped me; I should have remembered that it appears to be based off of a hedgehog.
Charmander learns Metal Claw, but doesn't become a steel type, so the same could go for Chespin. But personally, I really don't see it going through the game and not becoming part ground like Torterra.
But then again, I was also sadly mistaken when Samurott didn't become part fighting... Which is actually kind of irksome, but I'm sure that they had their reasons, just like they don't allow some Pokémon to learn certain moves even though it would seem like it would make sense that the Pokémon would be able to learn it.
But, also, did you happen to come across my list of Pokémon that don't follow their themes through and through? It's the majority of the starters, actually. It was an entire tidbit explaining that a Pokémon does not have to be one thing exactly, but can be loosely based off of a multitude of things. Chespin could probably be a hedgehog AND something prehistoric. The mole-like/lizard-like tail just screams proto mammal as well as ground, also supported by its coloration, but that's my opinion. Just because a mammalian Pokémon has a lizard-like tail doesn't mean it's a dinosaur, I get that, but some things in the designs are being placed there for a reason.