He can drop most things in the game, unless it is considered unique to another boss or quest. Obviously you will not get a Teeth of Terra or a Blockhead from Pete, but you could get a pitchfork from him. If a weapon says: “obtained randomly from any suitable loot source but has an increased chance to drop from x” he can drop it. He also drops many so called unconfirmed or (unassigmed) drops, like: Unkempt Harold, Shreddifier, Madhous, Fire Bee, Pandemic, ect with better odds.
His loot pool is massive, really only missing unique style weapons regardless of actual rarity, and Seraphs not part of his DLC. I know they say he can drop Seraphim weapoms but Ive never seen him do so.
What is a man?! A miserable little pile of secrets!