More monstery/cryptid stuff
You should have known taking food from humans was too good to be true.
You were once lithe, powerful, fit, a creature beyond human capabilities and understanding.
And it wasn't like you'd been hurting for food, you were an excellent hunter and forager, you just... got greedy.
A scientist offered you half a burrito once when she came across you in the woods, and you found yourself quickly addicted. Soon, raw meat and small berries weren't enough for you, you had to sneak into town and dig around in the garbage like a common animal hoping to find a scrap of cooked, seasoned, prepared food.
It made you a target, you were seen too often, and you were lucky no one hunted you down. The humans were far too interested in studying you, if only they hadn't noticed your weakness for their food!
The human that would become your caretaker- and jailor- was the first to use food to pacify you.
They'd lure you in with the smell of fried pork chops and mashed potatoes with cheese, convince you to stick around for blood samples and measurements by offering you a bucket of macerated strawberries, then feed you egg rolls and rangoons until your full stomach lulled you to sleep in the lab.
You managed to tear yourself away now and then, return to the woods and caves with your dignity (although it was a tighter squeeze into those caves these days) but you always found yourself returning whenever your stomach growled.
You got lazy. Hunting and digging up roots was hard, sticking your face in a tub of ice cream was easy. So what if your stomach ached like it might burst as your tongue cleaned every crevice of the fifth tub? So what if you were out of breath by the time the scientists led you from the front door to the lab? You deserved a break, and you'd lose the extra weight soon enough once you stopped indulging the humans.
You weren't concerned, not at all, not even when your stomach started dragging on the ground as you walked.
It wasn't until they showed you the enclosure they'd made for you that you realized with fright that you'd gone too far. They were taming you.
You tried to quit human food cold turkey- but even that just made you think of hot turkey served with stuffing and bread. You were waddling back to town before you even realized it.
Now you're a specimen, a lab rat, and since the humans got all the data they needed on you already they had to find a new part of you to study. You gave them the perfect idea too with your actions: how big can one creature get...?