Interesting. The art for the foulspawn certainly resembles the art for the ushemoi: a skinless hulk, a wrinkly wizard-like critter, a dude in a cape with four hands that joint at the elbow...
Though gutting old fiction isn't out of character for 4e, the 3e ushemoi are not related to the Far Realm in the slightest, and have a completely different mechanical hook. Fiction-wise, the ushemoi are forest-dwelling mercenaries for other evil creatures. Mechanics-wise, they are sort of masochistic: they get stronger or faster when they get hurt (though the sorcerer-leaders get more arcane power as they cast more spells).
In retrospect, the ushemoi fit extremely comfortably in 4e's roles, so I wonder if this critter was one of those late-3e attempts to float ideas that would become a core part of 4e, like other elements of the MM5 (they certainly aren't the only ones there that fit roles nicely).
Interesting, though. About the only thing they share with the foulspawn is the art...