Magpies, a bird that has a stealing attack; unlike monkeys, the stealing attack is limited to shiny objects (possibly based on their material) that are under a certain weight. Under that rule, rings, gems, and amulets are common targets of theirs. They probably have a rather fast speed, too. They should likely generate in small groups.

Magpies would have an alignment of 0 and lack the M2_HOSTILE flag, and thus may be peaceful to neutrals, but angering one makes all other magpies angry as well.