Fantod is a hand-puppet on the Weirdo's right hand that speaks. The Weirdo did not acquire Fantod during his time at the Weirdo Academy(?), but after his assignment to the Mansion. He won Fantod in a skipjack game in Bascomp, on the way to the Mansion (as noted here).
Fantod has no resemblance to any particular species. He has a bow-tie like the Weirdo, a long nose and two downward-hanging ears. He has two arms, most likely the thumb and little finger of the Weirdo.
It is currently unknown whether Fantod is a truly independent entity, an expression of the Weirdo's craft, or something in between. It may be that the Weirdo is a ventriloquist and was inspired by the puppet he won to be a last-minute addition to his Weirdoing. However, Fantod speaks to the Weirdo addressing him as another person, even when nobody else is around. Fantod speaks coherently when the Weirdo is drunk or half-asleep.
The exact functions of a Weirdo are unknown. He may do the human equivalent of Finagling, helping the various residents of the Mansion in various odd ways.
WARNING MINOR SPOILERS!
The Weirdo also appears to be studying the Mansion itself. He has been known to go through the logs of previous Earls in the Mansion's library. He has also been carefully and covertly observing the Basement and its residents, interacting with them in odd ways (like here in the last strip, where he uses a puppet). He has given a report of his observations to an unofficial visitor from the Weirdo guild to whom he has given a report of his discoveries.