You remember correctly - it's from the second book, "The restaurant at the end of the universe", and the animal in question had been genetically altered to strive for its own consumption, and enjoy the whole process of it.

It's quite a terrifying thought, to be honest - maybe because the animal in question is very much conscious and intelligent.

I'm never quite sure whether or not Douglas Adams wrote it for comedic effect or to provoke some deeper discussion.

Might as well be both.