Negaduck is a name shared by two characters (or possibly one) from Darkwing Duck.
While battling Darkwing Duck, Megavolt accidentally blasted him with his Tron-Splitter device, separating Darkwing into his positrons and negatrons. The positrons became the good Posiduck; the negatrons became the evil Negaduck.
Negaduck imprisoned his counterpart and Darkwing's companions, then sought out Megavolt. He intended to destroy the Tron-Splitter to eliminate any chance of being re-integrated with Posiduck. In Megavolt's apartment, Negaduck tried to attack the villain, only to be accidentally blasted again by the Tron-Splitter. This "galvanized" him, turning him into a powerful negative-energy being. Negaduck wreaked havoc on St. Canard, until Posiduck was also "galvanized." In this form, he held Negaduck off long enough for Megavolt to re-merge them into Darkwing Duck. The Tron-Splitter was destroyed shortly afterwards.
The evil twin of Darkwing Duck, Negaduck's origins are unclear. It is presumed that he is from the Negaverse. Negaduck is probably Darkwing Duck's deadliest enemy, for he has no agenda besides wreaking chaos and havoc upon St. Canard.
On one occasion, Negaduck did all he could to destroy the city when he discovered that he was Public Enemy No. 2, rather than No. 1 (which was held by Doctor Slug). In his continued efforts to destroy Darkwing Duck, Negaduck gathered together the Fearsome Five, which consisted of Megavolt, Bushroot, Quackerjack and the Liquidator.
Despite Darkwing Duck's efforts, it's not likely he'll ever be rid of Negaduck. He has always returned, even from oblivion itself.
According to Tad Stones, the second Negaduck was supposed to be one and the same as the original. An episode explaining this was planned but never produced.