In section M4 the party encounter an otyugh polymorphed into an ogre. The encounter is straight to combat, so there is no real way of knowing it is anything other than an ogre, but a dispel magic reveals the true form.

I don't know 3rd edition, and am reading this because I am trying to convert it to 5e, but what is the relevance of the polymorph, is there something I am missing as to why this isn't just an ogre, and how are the party supposed to find this out at a time when it could be of any use?