Text (UG+errata): Enchanted creature loses all abilities and is a 1/1 creature that counts as a Chicken. [Oracle 2009/07/08]
The "is a Chicken" text overrides the creature's other type(s). [QAS 1998/09/09] It does remove Legendary status if the creature had it. See Rule G12.9. [bethmo 1999/04/10]
The card keeps all its other characteristics. [Barclay 1998/08/13]
All tournament formats have banned cards from Unglued.