The Chickencreature spell was cut from the original Dungeon Keeper, likely due to being unbalanced. Not much is known about it, but references are made to it by the Dungeon Keeper Gold manual:
"26.2 CHICKEN (WEAPON)
When fired at enemies, this turns them into chickens. The enemy isn’t able to attack but has the same strength and thus won’t be as easy to kill as a normal chicken. This spell wears off after a while. Acts the same as the Turn To Chicken spell on the Research Panel of the Control Panel"
— Dungeon Keeper Manual
From this, we know its effect would have been identical to the ChickenKeeper spell, but would've been fired as a projectile (possibly using the same sprite and many other projectiles such as Poison Cloud and Freeze, with a possibility to have launched an egg.)
Curiously, despite being cut from the game, this spell is also documented in NTT Publishing's Dungeon Keeper Premium strategy guide book, and the Unofficial Professional Guide.
Despite being immune to its Keeper counterpart, Avatars are not immune to this spell, at least in KeeperFX.