MMPR vs. Knightmare

Dairanger's Lieutenant Shiryuu was not a monster per se (in fact, he's just an ordinary person wearing a suit of armor), but was instead a victim of the classic "misunderstanding between heroes" trope. Unfortunately for him, this time didn't end with a team up at the end, but rather Shiryuu's death. The Dairangers didn't find out until afterwards that he was actually helping Master Kaku with his secret plan to defeat the Gorma and they'd just made things worse.