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.
Wait do you know what level adjustment ment? Look, level adjustment was how you played those races, Say I wanted to play a Teifling, that race was level adjustment +1. The racial abilities were equal to a class level on top of base race. So you would skip a level or take the next level requirement for that class to reach the new class level.
