Yeesh, can't believe nobody else realized it at first reading as I did. I thought it was at least kinda well-known, and am fairly sure it was in one or more older editions too.
Your monster is the Julajimus, reprinted for 3rd Edition in the Monster Manual II (dunno if a 3.5 version was printed later, but the errata for MM2 or the 3.5 revision update PDF probably have it).
Julajimuses are huge aberrations that look like strange ape-monsters, kinda like a baboon, with nasty teeth and a reptilian tail. It can polymorph at will into small, cuddly animals as a free action, to draw in hapless prey before transforming into its full, terrifying size. It's got stunning, deafening, and frightening abilities, immunity to Enchantments, and sunlight vulnerability, along with some resistances and darkvision.