Why are they always carnivores?

When Professor Montague found the cow in his kitchen, he assumed it was just an end-of-semester prank. The animal looked confused, but docile. It had scuffed up the linoleum, but hadn't done any harm otherwise. So he led it out into the back yard, and called the the police - and then Animal Services - to take care of it. It was at least an hour later before he realized that Domino, his black and white cat, was missing. At the time, he assumed that the cat must have escaped while the cow was being brought in.

Domino showed up two days later; the animal shelter called the vet, and the vet called Professor Montague. He retrieved the cat with a certain sense of relief, and a few curses in the direction of whoever had let Domino escape. The cow, he could live with - he thought - but losing his cat was more serious. The police weren't much help. They didn't have any idea who had brought the cow into his house, or how. Come to that, they seemed to be a little confused about what had become of the animal afterwards. Realizing that he would probably never get the whole story, the Professor went back to writing articles and did his best to forget it.

A month later, he woke up with a cow lying on top of his bed. For several minutes he remained convinced that he was actually asleep and dreaming. It was only after he'd managed to worm his way out from under the covers, and put in his second cow-related call to the police, that he decided this was actually happening. Given the damage to his bed, and the condition of the animal - it didn't seem any happier about this than he was - he would have preferred to be having a nightmare. To cap the matter, Domino was missing again.

The professor spent the rest of the night wondering how it was even possible for someone to get a cow into his bedroom (let alone onto his bed) without his noticing. The man from Animal Services was even more puzzled; he called the next morning to tell Professor Montague that someone had apparently broken in to their facility, taken the cow, and left the professor's cat. "If it's a prank," he said, "it's awfully elaborate." Professor Montague agreed, and went to pick up Domino.

By the time it happened again, he had changed all the locks on his house, installed an alarm system and motion-sensitive outside spotlights, and taken an active role in the local Neighborhood Watch. He could not imagine who might be doing this, or what point they might be trying to make. He was only a little less puzzled about how they had managed it.

This time, he was standing in the kitchen. This time, he saw Domino twist and grow. He saw the cat's fur shorten, the teeth flatten, the bones change shape.

He saw the paws become hooves.

He saw, and he could barely believe it. His first thought was that he must be dreaming... But nothing else happened. There was a cow in his kitchen, looking at him. His second thought was that he must have lost his mind... but surely the authorities would have noticed if they'd been collecting imaginary cows. It was the utter, surreal mundanity of the scene that finally convinced him... and then he started to laugh.

Professor Montague lives in a small house, with a cat named Domino. Once a month, when the moon is full, he takes the cat outside and locks it in the small barn in the back yard. On the rare occasions when someone asks about the barn, he says it's a storage shed. He never talks about Domino, and he never tries to document the change. When he comes back in the morning, he takes the cat back inside the house. Domino usually spends the rest of the day sleeping; but then, that's how Domino usually spends his days.

You can blame my wife for this one. Seriously, though, why are were-beasts always predators? And why are the default forms always human?