Question about animals' movement

[Laharal]

I think I have heard somewhere that animals in their native environnement (forests, hills, mountains) do not have penalties for difficult terrain. Is that a homerule, Unearthed arcana, in my head, or just common sense?

[Greenish]

I can't recall seeing that rule anywhere.

Anyway, many animals are actually inconvenienced by difficult terrain (to some extent), which is why they usually keep to their paths when they can.