>>55918223I like to think that for rock and steel mons (at least, the ones that are more animal-like), they aren't actually made of stone or metal, they've just got really tough skin, so it seems like they're rock-solid. Besides the bits of armored plating, of course, like the white iron on Aggron.
So yeah, like the other anons said, it's likely soft, but firm, especially if the mon is on the leaner side. But if it's got some blubber on it from eating too much metal, then, well, it's probably not that hard to squish and mold to your liking. Just be careful, though, they're real heavy mons.