the Jester
Legend
Well, sure, but that's a circular answer. They 1e authors had to make the decision about why they should speak, too.
I would guess Tolkien and, in general, all the mythology and folk tales involving talking to giant animals. If you're looking for actual logic, I don't think you'll find any- in a lot of cases, D&D draws on a lot of tropes and myths and stories in which you see people that do it, so the game has it in there somewhere.