By footprints:
* Mammals: Dogs, cats, bears, deer, rabbits, squirrels, etc.
* Birds: Some birds leave tracks in mud or snow.
* Reptiles: Snakes, lizards, turtles
* Amphibians: Frogs, toads, salamanders
By other means:
* Snakes: Leave scales and sometimes slime trails.
* Insects: Ants leave pheromone trails, caterpillars leave silk trails.
* Worms: Leave burrows and tunnels.
* Slugs and snails: Leave slime trails.
Specific examples:
* Deer: Leave hoof prints and sometimes droppings.
* Snakes: Leave scales and sometimes slime trails, as well as sheds of their skin.
* Squirrels: Leave tracks and sometimes droppings.
* Ants: Leave pheromone trails that guide other ants to food sources.
Can you give me more details about the kind of trail you're interested in? That will help me give you a more specific answer!