[alloysteel]

Giant platypus, maybe? I understand they are also oviparous, yet are considered to be mammalian (they produce milk-like secretions from their bellies).

[Rudder]

I'll go for it being an egg-laying shrew.

[Bernard Marx]

Giant platypus, maybe? I understand they are also oviparous, yet are considered to be mammalian (they produce milk-like secretions from their bellies).

They are also unique (or were until Hillary came along) for being the only venemous mammals.