Pentagon is in fact, a spider-type changeling. Been one his entire life, actually. When his boyfriend is a butterfly, it generally makes it even harder for Pent to keep his obsessive, predatory personality under control on his molts.

To warn Oculus ahead of time, Pentagon must make it clear that he's actually a spider. That the form he uses isn't his actual form, and so on and so forth.

Oculus being oblivious as Celestia's sunbutt doesn't exactly help at all.