A Humanoid needs the Natural Spell feat to use spells if polymorphed into a non-humanoid form. Do you think that a magical beast would need Natural Spell to use magic if polymorphed into a humanoid form?
This isn't a rules question, as this isn't something addressed by the rules, more of a thought exercise about what changes when your "normal" is being something like a Unicorn or an intelligent Owlbear. Surely they learn to cast using their specific body configuration the same way humanoids do, so would they not be equally inconvenienced to find themselves in what -to their understanding- would seem like a clumsy and awkward body? Would not what is optimal to us be cumbersome to them?