A fit horse is a horse that is in appropriate physical condition to preform at a required level of work.
Certain horses on becoming too fit, may become overly excitable and energetic if they are not kept in an appropriate level of work so as to release the energy available due to their high level of fitness. Such horses may need to be "let down" until the horse has readjusted to being suitable for a less physically demanding level of work.
Scope generally refers to jumping, where it describes a horses overall athletic potential or ability, to jump well.
A fit horse is one that is in shape... race horses, endurance horses and three day event horses should be fit. When horses are "fit" they might have a lot of extra energy. For some horses that will make them hard to ride. This might cause them to buck, spook or run off with their rider. You might want a horse not to fit if they are being ridden by someone who is a beginner, is timid, or otherwise can not, should not, or does not wish to deal with that kind of behavior... also certain disaplines prefer a horse that carries more "meat" on it (hunters, or western pleasure for instance) so for those disaplines you might not want your horse to be too fit.
Scope is the ability to jump both high and wide and from various distances. Horses that are talented jumpers are said to have scope.
Years ago I rode a horse in a lesson that my trainer had just brought back from the jumper circuit (ridden by my male trainer). My trainer said that this horse was VERY fit. Being naive I thought, well that's good. Oh my gosh! I consider myself a pretty strong rider, but at one point I was cantering this horse and I could not stop him. Trainer had me ride a smaller and smaller circle. He actually got faster! I was kind of laughing and kind of not. I finally kind of had to just let go and he stopped. I just lost all my strength and was giving up. Then he stopped. But I now appreciate a horse that is TOO fit!