This was posted on the groaners listserv. The author is unknown.

The owner of a large rabbit became worried when his pet began to lose weight. None of the remedies of his veterinarian seemed to work. A friend suggested he ignore the prescribed nostrums and use goat’s milk to fatten the bunny.

So the owner went to a nearby tallow factory where a young goat dwelt and lived off the fat drippings. Milk from this goat was fed to the declining rabbit, but it’s weight loss accelerated and its health began to deteriorate alarmingly.

He rushed back to the vet who asked what he had been feeding the rabbit.

“Aha!” the vet exclaimed, when told, “That’s the trouble. You shouldn’t have used that greasy kid stuff on your hare.”