What does Ichabod see that makes him so terrified that he send his horse into full flight in "The Legend of Sleepy Hollow"?

In "," Ichabod Crane is riding back
home from the Van Tassels' farm when he sees a horseman "of large dimensions" mounted
on a large, powerful black horse. Ichabod tries to ride on ahead of the dark horseman then to
lag behind, but the horseman keeps pace with him. When they reach higher ground, Ichabod sees
the huge figure outlined against the sky and notices that this is no ordinary rider:

Ichabod was horror-struck on perceiving that he was headless!but his
horror was still more increased on observing that the head, which should have rested on his
shoulders, was carried before him on the pommel of his saddle!

This terrifying vision drives Ichabod to shower his horse with kicks and blows, goading
him on to go as fast as he is able. The horse now seems to be possessed by a demon and gallops
straight past the turning for Sleepy Hollow and toward the church. Ichabod struggles to stay on
the horse and is finally dislodged when the horseman throws what appears to be his head directly
at him. The next morning, a shattered pumpkin is found beside Ichabod's hat.

Even when utterly terrified, it is difficult for Ichabod to send his horse into full
flight, and the mysterious rider is able to keep pace when he does so, since Ichabod's horse is
so decrepitthough it has the remains of a fiery temper. The horse is described in some detail
earlier in the story:

The animal he bestrode was a
broken-down plow-horse, that had outlived almost everything but its viciousness. He was gaunt
and shagged, with a ewe neck, and a head like a hammer; his rusty mane and tail were tangled and
knotted with burs; one eye had lost its pupil, and was glaring and spectral, but the other had
the gleam of a genuine devil in it.