[Gothmog]

Here's a wild theory (one that I don't believe in myself, but it's interesting to speculate ): We know that the orcs was made as a mockery of elves, and at least partly from tortured and twisted elves. The exact source of the orcs hasn't been proved if I remember earlier discussions correctly. There's multiple factors behind the orcs, but this is one. We also know that trolls was made as a mockery of Ents (again, not completely proved but a highly accepted theory). Is it possible that some of the Entwives was twisted into trolls, like the Elves was turned into Orcs? Presuming Sauron had the power to do that of course.

The poor Entwife-slaves was also used as lab rats and was subjects to Saurons twisted experiments! And you feel sorry for the Ents that was left without wives!?

The only way to know where the entwives went is by solving this riddle I guess. If there even is an answer...