It was a regular turkey that was enlarged after Sarah Ravencroft cast a spell on it.
It's intelligence was originally on the level of any regular bird but once Sarah used her magic on it, the Turkey became very intelligent and under her evil influence, also extremely hostile. However, once Sarah's magic wore off, the Turkey lost its hostility becoming quite friendly. It's newly given sentient was also exploited by Scooby and Shaggy as they pretend they wanted to cook and eat it which caused the Turkey to become terrified and run for it's life.
Powers and abilities
When trapped, it revealed it could stretch its neck out.
Scooby-Doo and the Witch's Ghost
Sarah Ravencroft summoned a turkey to get her spell book from Scooby-Doo and Shaggy Rogers. That only made them laugh until they realized the turkey grew to a huge height, and began chasing them. Scooby and Shaggy scared the turkey by pretending to want to baste and stuff it for Thanksgiving. The turkey, scared, ran away.
- DTV2. Scooby-Doo and the Witch's Ghost (no lines)