when Ichabod sees something huge and black by the brook,why doesn't he turn and run away? what two things does he do instead?

Ichabod knows that his horse, Gunpowder, is no match for a demon horse:

"What was to be done? To turn and fly was now too late; and besides, what chance was there of escaping ghost or goblin, if such it was, which could ride upon the wings of the wind?"

So he tries to show some courage by stammering out "Who are you?"  He received no answer from the horseman, so after asking it again (and again receiving no reply), he started whistling a tune and prodding Gunpowder forward.  The Horseman stayed right with him until finally Ichabod got Gunpowder running toward the bridge.  It was Ichabod's hope that if he could just get across the bridge and reach the church he would be safe.