This entire "smart forgotten beast" thing reminds me of a weresomething vampire I had in my fort. Every month when the moon was full, he would go down to his tomb (at one point he had been the only survivor in the fort, at another point he became mayor, and I gave him a tomb to use, because noticeable residents get tombs dammit) and Turn there, where no one would see him, and he wouldn't bite anyone. Then he would return to the living levels to be useful for the rest of the month (I could be wrong, but I get the idea that among other things he was a mason and a chef).
He was too smart, and I couldn't work out what was going on for ages, until something disturbed him on the way to his monthlies (possibly the fort being locked down due to attack?) and he didn't succeed in transforming where no one else could see him.
I don't know if it was because he was both a vampire and a werebeast, or because he was a goth (Morbid set of opinions on his chart, odd set of favourite things as well, and did not value companionship in any way) or because he was not a young vampire, or what. But some dangerous characters in this game seem to get a set of traits that make them almost... safe. This one was my very own reformed vampire. Yours was a waterdwelling monster not unlike outside the gates of Moria. Peculiar. Gods I love this game.