Ducklifors was the incantation of a transfiguration jinx that transformed an organism into a duck. Fourth years and above were able to cast the spell, meaning it was likely taught in fourth year Transfiguration class.
Casting and effects
The caster would slash their wand downward to point at the target; a banana-yellow jet of light would shoot from the tip, with what appeared to be a duck made entirely of brilliant yellow light soaring in front. When it hit the target, there was a burst of yellow light and when said light faded, the target was in the shape of a duck.
From the English noun "duck", with the -fors suffix attached.