The GM should be able to apply a custom status that changes character appearance (e.g., like turning a character into a chicken, or any other race or monster).
I agree, they should, but this functionality is presently not in the game. "Chicken Form" is the only 'effect' you can add to a PC or NPC and there's no way to change the chicken form to an alternate form.
I wish it was just 'change form' as the effect and then you could select which form you want to change the player's appearance to with a drop down list under that. That way you could turn the player into a werewolf, a sheep, or whatever.