Do you think Bendy's horns change shape a little on certain occasions? Like they fold down like a cat's ears when he gets angry or they shoot up all pointy like needles when he's startled? Might add to his cartoony expressiveness.
I feel like they’re absolutely subject to changing shape, just like any other part of him – after all, he is a cartoon! But I’m not sure I understand the inclination to treat Bendy’s horns as though they’re kittycat ears. After all, they’re… not. They’re horns. Horns are made of bone, so they’re gonna be relatively rigid – about as rigid as the rest of him.
Basically, the way I see it, since they’re horns, they’d be as rigid as his skull. They’ll do whatever the rest of him is doing. He might be able to treat his set of horns as a hat, or perhaps move them individually with his hands…
…but while he may be able to interact with them separately from the rest of his head, well. Unless he’s involved in a gag that is rather literally comparing him to a cat or something, I can’t see his horns moving independently from the rest of his head as though they’re ears. Horns don’t have muscles in them, after all.
Basically, he functions on the same kind of toon logic as any other cartoon – his body probably follows certain rules pretty strictly unless specifically required to do otherwise for a gag.
That’s just my take on it, though.