Diablo III – Does Rakinishu’s Blade Have a Special Purpose

Larson is a "unique", or a specific, named version of a regular monster. Unless quest-required, uniques' titles are apropos of nothing (Easter egg additions). Uniques sometimes have special abilities (like rare monsters), but Larson doesn't. There are no other "eaters of vegetables", but maybe that's because none spawn so close to a delicious garden:

As for the joke bit, it may be a connection back to the "eat your vegetables" subliminal message in Diablo 1.⁵

Since the secret level and the method of reaching it has been revealed (the recipe requires Wirt's Bell), it seems like Wirt's Original Leg is just an in-joke.