Let me think... Also I count Demons as Semimegabeasts... mostly because I don't know what they are classified under.
Good (Semi)Megabeasts
-Note: These are Megabeasts that hang out in "Good" areas, not Benevolent.
-Fairy King/Queen: She isn't too tough but she has an army of Large fairys under her control and can annoy your fortress with Fairy swarms (Which are already in the game as Pests). Is capable of extending good boarders.
-Ambrosial Queen Bee: Similar to the above, she is the Queen of the swarms of Giant bees.
-Ambrosial Butterfly: A Giant Butterfly that uses poisons to suppliment its strength.
-The Pure: A being of such "Good" that it cannot help but to attack and destroy nearly everything in the world. It isn't intelligent and attacks on pure instinct then one any sort of Philosophical grounds.
Benevolent (Semi)Megabeasts
-These are the goody ones or at least generally helpful to other civilisations
-Angels: Naturally Benevolent, dislikes Demons. Though it can make other cities mad by stealing what it needs to live or attacking those it finds offensive. The Exact name isn't too important.
-Stone Guardian: A Statue carved that protects the town it is built in
-Little God: A Semi-Megabeast that devotes itself to the protection of a single family line.
Normal (Semi)Megabeasts
-Queen Antmen: A Stronger then usual Antmen who is protected by others as well as the few Male Antmen in the colony. She is actually Worshipped by the Antmen.
-Roc: A Giant bird
Evil (Semi-Megabeasts)
-These live in Evil Lands
-Undead King/Queen: Brings things back from the dead and can extend evil boarders.
-Amalgamation: Several undead put together as one ready to kill.
-Hundred Ghost: Several Ghosts working together as one being.
-Feeder: a Being who feeds on the dead. Though seemingly helpful it also will eat the dead of other civilisations who may or may not think it was such a good idea.