This took a while to find, but in a previous thread, I went through the 5e races that used to be monsters in 3.5
Monster RacesThere were several books that covered making those monster races playable - including a book dedicated to just generally telling you how to do it so you could play as whatever.
It should be noted that Savage Species was among the worst D&D supplements made, as it was designed specifically with the intent of discouraging its use by overly punitive Level Adjustments. "Here's a book to play monsters, but since we don't want you playing monsters, they will all suck".
Then again, the same thing could be said of anything not a caster in that garbage edition.
