Driders are former drows that have the lower body of a giant spider, much like a Bariaur, except the spider and the drow part. They weren't born that way, but Lolth didn't like them as regular dark elves so she changed them to driders instead.
Source
Monstrous Compendium Volume 2 (TSR 1989)
Ecology of the Drider, Dragon #312 (Paizo 2003)
- 525 reads