One Template. One Class. One Amusing Critter.
Half-Fiend Ibixian Cleric, with the Death and Evil Domains. Give him a big mace,and a decent bluff/intimidate, and have him claim to be the Avatar of Orcus. A few undead minions would also help the ruse. (Ibixians are in the mini's handbook I think, but any goat-looking thing should work).
Awakened Phantom Fungus Druid It's invisible, it has an animal companion to tank for it, it can cast summon nature's ally spells from hiding without giving away it's location. Have it throw entangle on the party and then summon flying creatures, or wildshape it into one and use Natural spell mess with them further. Have it target anyone who might be able to cast See Invisibility first, of course.
Half-Fey Formian Warrior Paladin of Freedom 6 When a colony of formians began encroaching on their forest, the Fey decided to fight back by undermining the formian's lawful nature, tainting an entire generation of their young with the forces of chaos. While the formians managed to slay most of these chaos-born, a few managed to escape and reach adulthood. One such creature decided to champion the cause of freedom, and now seeks to topple oppressive lawful regimes wherever he may find them! As a creature of chaos, he probably has no qualms about using his various spell-like abilities (including lesser geas 1/day) to try and "recruit" others for his cause. (Half-Fey is from Fiend Folio, Formian Workers from the MM/SRD, and Paladin of Freedom from Unearthed Arcana/SRD).
Seriously though, with his base stats of Str 17, Dex 18, Con 12, Int 10, Wis 14, Cha 15, one could make a decent Sorceror, Bard, Cleric, Barbarian, Rogue or Ranger. While they're not incredibly tough, they could be an interesting challenge, especially with a mix of classes (having a caster cohort to buff the paladin above for example). Plus, they're 10HD chaos-aligned, Butterfly winged Ant-centaurs, with spell-like abilities...what's not to like?
Half-Illithid Lizardfolk Sorceror or Wilder 6 Just take the Half-Illithid Lizardfolk sample creature from the Fiend Folio and add six levels from the SRD.
Ti-Khana Deinonychus Rogue 5 "It's just the two raptors, right?" "Unless they've learned how to Open Lock and Disable Device." "..." As an added bonus, you can drive any dinosaur "experts" in your group nuts by having them flick out their snake-like tongues a la the test footage for the velociraptors in the first Jurassic Park that was scrapped (but is on the making of special). (Ti-Kahna Deinonychus from FF)
Half-Troll Flamebrother Salamander Fighter It's a flaming, fast-healing, small-size snake person with an 18 strength, improved grab and constrict that specifically works on things larger than itself, rend with it's claws,and deals fire damage on every touch. Plus it has an AC of 24 before equipment. Not bad. (Half-Troll from FF, Salamander from the MM/SRD)
Skulk Vampire Rogue 3/Assassin 3 Yeah, I know it's two classes, but with only two monsterous humanoid HD, it needs a base class to get the skill ranks to qualify for Assassin. Continous non-detection and a +15 racial bonus to Hide checks is just so tempting... (Skulk from FF, Vampire from the SRD)
Robert "Guess Who Borrowed the Fiend Folio from a Friend?" Ranting