Post a picture of him (make sure it's reasonably well-focused and exposed!)

Exotic frogs and lizards (even small snakes) sometimes come along for a ride to the nursery. And since some plants come from across the border or over the ocean, you can get some strange little visitors.

My son was at the nursery the other day, and a frog jumped out of and then back into a plant. The person at the register said "don't worry, we don't charge extra!".

So I wouldn't count on that frog being a native species. It could even be an American species, but one that requires a different environment than your own.