When you use mutate, the spell you're casting is still the original creature spell in every way. It is still a creature spell, of that name, with the CMC of the original card (not the mutate cost). You will draw a card off Beast Whisperer for it, for example.

Is mutating a casting?

Casting and resolving creature spells with mutate

A spell cast with mutate becomes a mutating creature spell. It requires a target creature with the same owner as the mutating creature spell. ... A mutating creature spell is a creature spell, even if it won't enter the battlefield as a creature.

Does mutate trigger on cast?

Yes. When your Illuna resolves and mutates on top or bottom of the non-Human you targeted, any and all "whenever this creature mutates" abilities will trigger. That includes Illuna's ability itself, as well as any abilities that were on the creature prior to Illuna mutating onto it.