The Evil Duck was a monster that resided in Henry's Tunnel.
In Sodor Short: Monster, The Evil Duck resides in Henry's Tunnel and tears up any engine that tries to pass it, as said by Donald & implied using Barry. In the short, The Evil Duck destroys Thomas, Percy, Gordon, Duck, and many others before being blown up by Diesel using a large railgun. In the Sodor Short: Bridge, The Evil Duck survives the explosion, and returns to kill the remaining engines after crossing the Black Bridge. Not much known is about that duck, however, but it is revealed in Sodor Short: Castle, that there are multiple ducks when released by the French Trucks. After World War II, The Evil Duck came and derailed Douglas while mourning for Lily.
Behind the scenes
The Evil Duck is not canon to The Stories of Sodor