The Mega Man Killers, also known as the MKN (Mega Man Killer Number) series, are the robots created by Dr. Wily specially to eliminate Mega Man.
In some sources related to Mega Man: Dr. Wily's Revenge, Enker is designated as a "Mega Man Hunter" instead of "Mega Man Killer". Quint is not part of the MKN series, but serves an analogous role in Mega Man II.
Each Mega Man Killers made separate debuts in different games. It was not until Mega Man V that the three robots are together. Although very powerful, Either Dr. Wily or themselves made a fatal mistake of taking on Mega Man individually and was destroyed again separately.
They also appear in hidden challenges in Mega Man 10 where if defeated it allows for Mega Man to use their weapons in other challenges or the main game. These challenges are unlocked either by purchasing them as DLC in the original release or by completing the main game or using the secret code at the MM10 title screen in the Mega Man Legacy Collection 2 re-release.