The game of D&D has changed a lot. Third edition and pathfinder had tons of books out focused on player options. The problem I have with those player options was that it seemed to complicate mechanic and rules in favor of selling more books to Players as well as DM’s.
If the players can buy and use the book you will sell more books. The DM buys the module and runs the rest through the game, but if the players all wanted a copy of the book at the game to use making player content is the way to go. I found it made the game difficult to DM and difficult to manage. Where I can run a game and know the rules and how things work with some editions, those that focused on this made things like combat more difficult and more time was spent at the table looking up rules.
AD&D had a select few classes and races to choose from. Second edition gave us splat books. If you are running the older mechanics in some form OSRIC/S&W/Labyrinth Lord/Old School Essentials there is a good possibility you are looking for stuff that is not as complex.
Myself, I really like books written for the DM and not players. The players become unique in game is my philosophy. Have you ever read the comic from the 90’s from Marvel Ravage 2099? Ravage was an evolving character. The adventures made him different. I like the idea of level 1 characters being a starting point and playing through the changes and events that makes the character unique. I have heard the term “Luke on the farm” to describe a level 1 character before they go through what makes them unique.
But today I am talking about playing monsters! This is more common with new editions, people seek a variety of non-human races. I play hyperborean where we only have humans, no elves or dwarves. Gary almost saw it as power gaming. In the Advanced Dungeons & Dragons Dungeon Masters Guide Gary wrote about playing monsters and saw the desire to play monsters as sort of power gaming.
Gary Gygax also wrote on that page of how he saw playing humans gave a shared understanding of reality and basis for the game as the outlandish fantasy elements we were exploring was the wonder and fantasy of game. Gary also said that age old cliché line about humans being the true monsters.
But enter D&D 2nd edition and we got a lot of books that people call “splat books”. This is where the Complete Book of Humanoids came in. If you are running an old school game I strongly suggest picking a copy up of the Humanoids book if you want to delve into new races and playing monsters. It gives the stats for lots of monster races as player races and packages for classes for monster races as well. If you are looking to play a retro clone this would give you some pretty close stats and a feeling of how you might go about rolling up monsters.
The original Palladium rpg was ideal for running monsters and horrors. I strongly suggest picking it up. I also might give a special shout out to the earlier editions before the second edition as a great game. Even the magic will cross into monstrous realms like Gary said about humans being the monsters. The Palladium Fantasy RPG was focused on playing anything and everything. That is what the system is really good for. So, why reinvent the wheel? Forbidden Lands has some great but limited monster races and is ideal for approaching running monster campaigns as well.
Addressing racism and oppression in your game might be great with playing monsters. It has been pointed out just how some of the language in fantasy games mirrors old racist, orientalist and colonialist language. Playing monsters that are described as this you can push back against that and you can explore a villain that is very real. I would caution doing that and be sure the group knows what is going on and you approach this with tact, respect and self awareness. Maybe read some books on racism and colonialism before doing this.
Look at what is said about the races in some of the material presented with a critical eye. You are creating a fantasy world and this fantasy world can be improved and you can explore it in ways that make it interesting and meaningful.