Here’s where 30 day challenge gets a little less edition neutral, as some of this taxonomy of monsters is really just used in later editions. Honestly I’ve never owned a 3e or 4e Monster Manual but I beleive an abberation is an unnatural monster in the sense that it doesn’t have a “normal” anatomy like an animal or even a chimerical monster like a griffon, but instead looks alien; some just have weird origins or abilities I guess based on the wikipedia entry. Going by that list, I see that some of my favorite monsters are abberations.
They really do fill my players with fear. Earlier this summer there were some madcap hijinks as the party’s front line fights cowered and fled from a lone rust monster, leaving the often-maligned back line to deal with it. Come to think of it, that adventure ended with a near TPK as the cleric, apparently despondent over the loss of his magical plate mail, walked suicidally into a hallway he knew was filled filled with yellow mold…. The most infamous and feared part of the dungeon in my campaign is surely the room with what appears to be a rust monster spawning device, which will dump a new rust monster into a large open chamber every round until someone pulls a lever on the far side of the room. Various parties have dealt with it in different ways, perhaps the best was sending in a naked dwarf with a club fight the rust monsters — cruel but effective.
Next campaign, rust monsters will look like baby seals.