So a gull, a lamb, a pigeon, and a monkey walk into a bar...

pigeon
slang noun
A person who is easily deceived or victimized: butt, dupe, fool, gull, lamb, pushover, victim, sucker, fall guy, gudgeon, mark, monkey, patsy, sap. Chiefly British mug.

(PS: a gudgeon is "A small Eurasian freshwater fish".)