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. Unfortunately, these monsters all use the same HD and LA of a standard monster, so anything that was over LAed (which was pretty much everything, but especially Ogre Mage) was kinda useless as a class. You also needed to take every, single, level of a monster class before you could multiclass. 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.
Since Savage Species was a 3.0 book, and really wonky, it needed a 3.5 update. A series of web articles that sorta updated the monster class rules, but mostly focused on adding templates to the progression rules. Among the differences to the original was that you now can multiclass out of a class before taking all the levels. In addition to the templates a couple monster classes were created for the DND Fight Club article series. Most notable of these was the Ambush Drake, which is unusual for a monster class in that it's actually a pretty good class. Ambush Drake gains an HD (of Dragon HD no less, which is actually pretty good) and useful class features every level, which makes it a pretty viable option for melee characters.