The Executor is an enemy encountered in Ninja Gaiden II and its remakes.
The Executor enemy appears to be a cyborg of sorts, with a human head connected to a machine. It is first encountered in the story chapters of NG2 during "The Flying Fortress Daedalus" chapter, not counting any Tests of Valor. The only attack of the Executor enemy is gunfire from its dual mounted machine-gun "arms".
Sentries are first encountered in the story chapters of NG2 during "The Heart of Darkness" chapter (not counting any Tests of Valor) after all ninja are killed in the camp with two spotlights. They are not encountered in Ninja Gaiden Sigma 2 as they were removed from that game.
The Sentry enemy appears to be a variation of the Executor enemy. Although the Executor enemies crawl on the ground with four mechanical legs, Sentries have no legs and appear to quickly hover over the ground with some sort of flying mechanism. The only attack of the Sentry enemy is firing missiles from its dual mounted missile launcher arms.