Track (in hunting) is used for fresh paw prints in soft ground, or other physical marks (eg. tire tracks). It is usually plural. It is also a verb, meaning to follow these marks. We tracked the elephant into the forest.
Trail is used (in hunting) for the prey's scent that the dogs can detect.
It is also used to mean a path worn by consistent use over a long period
The words "trace" and "track" would not be right here. Hounds follow a fox by scent, they don't look for footprints (track). Only humans would follow the track of a fox.
"Trace" (of scent) might be used here but the article would have to be changed: "The hounds found a trace of the fox." Only humans can find "the trace" of the fox (bits of fur, vestiges of footprints, bedding areas, e