The Bullchicken is a Bullsquid from Half-Life, slightly modified to have different sounds and model.
The Bullchicken's sounds were all replaced with Quotes from Billy Mays' Oxyclean Detergent Commercial where he says "Are you on the ball?" And in Crack Life Campaign Mode, the voice was tweaked to have a higher pitch. The Model appears to have a Chicken on its back from CS 1.6, Originally from Quake 2.
Bullchickens are hostile with each other, if one Bullchicken meets another, they will immediately start attacking each other until one dies.