As I mentioned in the slime dragon write up (which needs to be reposted some time), I have been thinking about non-humanoid monsters that are witches (or shamans or other spellcaster) with patrons. The best sources for making such pacts are Distant Horizon's Eclipse and an online (free pdf) supplement for the Second World Sourcebook. Both provide costs in the form of actions, parts of the character (like a shadow or passion) or other unpleasant things. There are plenty of sources for patrons, from Goodman's DCC rpg to Sine Nomine's Black Streams: The Pacts of the Wise (a short pdf of just patrons and what they can do). For my example, I use the latter (though I have modified the benefits and penalties a bit).

The ice forest dragons are white dragons witches with the patron Ikrikik, the spirit of insects. The spirit was contacted by the witches' ancestors when they were facing extinction. Ikrikik is concerned with consumption and reproduction and it is the latter that got the dragons' attention. After forming the pact, they were made fecund and a female witch produces about triple the normal number of hatchlings. That the dragons were made allies of insects, including most giant vermin, wasn't very important to them then. That has changed.

Now the dragons have used their innate magic to grow a forest of ice and snow so that magical vermin can consume it. They hunt constantly to feed both their brood and the carnivorous insects that Ikrikik sends to colonize the frozen wastes. The dragons even have a poorer armor class as they shed scales in lean times to feed their insect allies (if the adults fail to feed them, the insects will consume the hatchlings). The dragons are common, poorly fed as adults (they must eat lichen encrusted stone) and are slowly expanding into the more temperate lands, sending a wave of ice monsters (like winter wolves) ahead of them- the monsters that do not want to be insect food.

A similar idea is based on the wizard kit Mystic of Nog in The Complete Sha'ir's Handbook. Those wizards lost some of their spellcasting ability in exchange for personal enhancement. Monsters with abundant spell-like, supernatural and spell casting abilities could do much the same. DM's could justify this with a patron, ritual or call it a mutation or aberration. In any case, it would be best served with the spellpowered template in Silverthorne's Deluxe Book of Templates 3.5. Cash in their existing powers and then use the formula in the template to give them permanent supernatural abilities. Balance is kept and the DM has a lot less spell flinging to keep track of.