Baron Duin Halfmorn is the werewolf. He is both a leader for the Doviello, and a unit that is buildable when you get Feral Bond. There is some type of code that is supposed to make it so that if the leader Baron is in the game, that the unit cannot be built by anyone, while he will be able to build Varulv, who is essentially a copy of the Baron. However, while the code does prevent the Baron (unit) from being built is the Baron (leader) is selected, Varulv is still not buildable by him.
If it help, you can find the stats for both in CIV4UnitInfos.xml, but I can't find the code that deals with whether or not he can be built.