There are three main types of Yuan-ti
Yuan-Ti Pureblood -CR3 - More human than snake, has very reptilian features.
Yuan-Ti Halfblood - CR5 - Humanoid, snaketail instead of legs, very snakelike.
Yuan-Ti Abomination - Large, CR7 - Like a snake/ogre.
Yuan-Ti Wretchling - CR 1 - Unintelligent twisted feral humanoids (Secrets of Xen'Drik)
Yuan-ti in My Eberron
In my eberron setting, the Yuan-ti were created by the Master of Magic, the Fiendlord Rakshasa, Lungtai. They were one of his 6 chosen races, sent out into the world to find the Fiendstone Constitution Shard. He raised them to be masters of the Eldritch arts, with many becoming Warlocks, although their snakeblood had the effect of also making some latent sorcerors. The Yuan-ti have a firm grasp on the workings of magic and in this setting Use Magic Device, and spellcraft, is always a class skill for them when they take character levels.
It is possible to encounter the standard Monster Manual type Yuan-ti on Khorvaire, usually in Q'barra. These are offshoots of a long forgotten cult which left Xen'drik, and Lungtai's service, millenia ago.
