Dynamo

Dynamo (ダイナモ) (not to be confused with Dynamo Man in the classic series) first appeared in Mega Man X5; he was sent by Sigma to knock Eurasia out of orbit. He appears twice--in both cases to interfere with the mission (whether it's getting the Enigma ready or preparing the Shuttle).

Dynamo appears again as a secret boss in Mega Man X6, in the Soul areas after Gate's Lab is unlocked and defeating both Nightmare Zero and High Max. Being struck by Rainy Turtloid's weapon causes him to be stunned and a follow-up hit from it makes him drop 200 Nightmare Souls, for a maximum of 600 per fight. Dynamo is a powerful, defensive, and fast opponent, although the ability obtained by Zero from the aforementioned Boss easily makes short work of him.

Dynamo can always be fought every time the player visits any of the Nightmare Areas, allowing X or Zero (possibly even both) to quickly farm Nightmare Souls.