The Hunter in Hades by Basil Hall Chamberlain

A handsome and brave young man, who was skilful in the chase, one day pursued a large bear into the recesses of the mountains. On and on ran the bear, and still the young fellow pursued it up heights and crags more and more dangerous, but without ever being able to get near enough to shoot it with his poisoned arrows. At last, on a bleak mountain-summit, the bear disappeared down a hole in the ground. The young man followed it in, and found himself in an immense cavern, at the far end of which was a gleam of light. Towards this he groped his way, and, on emerging, found himself in another world. Everything there was as in the world of men, but more beautiful. There were trees, houses, villages, human beings. With these, however, the young hunter had no concern. What he wanted was his bear, which had totally disappeared. The best plan seemed to be to seek it in the remoter mountain district of this new world underground. So he followed up a valley; and, being tired and hungry, picked the grapes and mulberries that were hanging to the trees, and ate them as he trudged along.

Happening suddenly, for some reason or other, to look down upon his own body, what was not his horror to find himself transformed into a serpent! His very cries and groans, on making the discovery, were turned into serpent's hisses. What was he to do? To go back like this to his native world, where snakes are hated, would be certain death. No plan presented itself to his mind. But, unconsciously, he wandered, or rather crept and glided, back to the entrance of the cavern that led home to the world of men; and there, at the foot of a pine-tree of extraordinary size and height, he fell asleep.