It means to get ahead of yourself.
It comes from a soliloquy in which Richard talks about his plans once Clarence and Edward are dead, but then he brings himself back to earth, saying 'I run before my horse to market', because they are both still alive and only when they are dead can he start counting his gains. It's a little like saying 'counting your chickens before they are hatched.'
Grendel is living in a place we can refer to as
hell (metaphorical), the place is inside mountains, a kind of cave with intense
smell and waste, which produces an impression of Grendel as a vicious and
insensitive entity. <span>
<span>As the origin of Gredel is tracked back to Cain who killed Abel
that is also adding up to the description of him being vicious, selfish and
insensitive monster.</span></span>
