Yesterday I talked about champing at the bit. Raring to go is another saying used to mean the same thing - being impatient, wanting to get on and do something.
Raring in this context is a slight corruption of rearing and it refers to the fact that some horses, when asked to wait before a run - when they know they're going to get to run - will rear up and even bounce a bit on their hind legs. They may also plunge, which is going into a half rear and then leaping forward.
(Not fun to ride, and I speak from experience).