First off I noticed it suddenly appeared in my pet flyout. It also said "Call stable pet 3" instead of any specific name.

Here you can see I should only have my toad pet active and on me

The worm doesn't have any abilities except the baseline pet ones. No family or specialization abilities.

And here you can see it appear on my character warband screen, in I believe it's original size

My current guess is that for some reason, WoW tried to 'find' the last pet stabled on my slot 3, and for some reason retrieved cached data or something server-side when this pet happened to be in that slot.
I'm actually more scared I can't properly abandon it and have that slot back tbh lol.
Edit: Just Beast Lored it. It has no stated Family, and is Ferocity