What does it mean when a monster is an "eater of vegetables"?
I just came across a unique Risen named Larson the Strange, that was denoted an "eater of vegetables" (where most uniques would have something that makes sense and/or is useful such as "knockback" or "teleporter").
Does this unique actually have a special ability (other than being a tad more resilient like most uniques), or is this merely some sort of joke?
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
It is just another Easter egg all blizzard games are full of things like this,especially diablo 3 as it would appear :)
Here is another monster with weird "ability"
Many of this easter eggs reference to someone or something but no one found anything about Larson yet.