Stagger Startling:

An alp-luachra, that seems to be like any other on the first sight. He loves to rest in bellies and sleep there too, like his fellow alps. However, he has a huge tickle for turning the tables: He's a predator alp-luachra too~

He grows when you feed him regular food, as he regulary eats insects.
He snug into a lab once and ate something there, that was infused with special substances.
After he digested it, it gave him that thing of, whenever he eats regular food, he grows, but the growth only lasts 5 minutes.
He has 5 minutes to devour his prey, before he shrinks again.

He begs others for food, mostly one that is alone. Whoever he manages to fool with his cute innocent begging face, feeds him and has to see the terror when he grows, quarter size bigger than the feeder.
A quick wrap of his tail around the victim and they're immobile. And then the real horror happens, he eats the feeder.

Once he's done with his meal, the prey cramped up tight inside him, he shrinks back down to alp size ( and prey inside shrinking along but still staying three quarter to his size). And at last, he then slips into a belly of someone else, to relax and digest his prey in said belly.