Savage Species

Savage Species was a Dungeons & Dragons 3.0e splatbook dedicated to a fuckton of alternate and variant rules for homebrewing playable races based on NPC monsters. It's basically the "Variant: Monsters as Races" section from chapter 2 of the 3.0e Dungeon Master's Guide (or chapter 6 of the 3.5e DMG), hopped up on bath salts and expanded out to a whole goddamn book. It introduced or at least popularized the notion of "monster classes", whereby players could start out as severely nerfed versions of extremely powerful monsters (like mind flayers) and then gain levels in that monster class instead of a normal character class as they leveled up, eventually turning into the normal version of that monster. It also made several monster-related changes to the rules, such as eliminating certain creature types and subtypes, changing how face and reach worked, and making monsters gain feats and skill points at the same rate as player characters. These changes were incorporated almost immediately into 3.5e, so the rules in Savage Species are sometimes referred to as "3.25e"

To pad out the page count, it also had chapters dedicated to new equipment, new prestige classes, new spells, and all the other bullshit that you find in every Third Edition splatbook ever. Does the "Survivor" prestige class have anything to do with playing as a monster race? No but fuck it, we're including it anyway.

Finally, it has not one, not two, but THREE appendices (yes, that is the correct plural form of "appendix"). They collectively make up a third of the whole fucking book, and Appendices I and III are dedicated to new monsters and monster classes, making one wonder why they weren't just made normal chapters.