From the folklore of the Baiga
The Pandava (five brothers) wanted to know what was underneath the earth, and they fell at the feet of Bhimsen and begged him to go down and see. So they tied him to a little seat and let him down by a rope. He went down down down for eight nights and nine days. Then the Pandava, out of mischief, let the rope go and he fell to the bottom. He began to weep, wondering how he was to get back. But the old cobra who lives there gave him the horse of lightning and he sat on it, and before he knew what had happened he was back home. He has never ceased wondering at his wonderful ride.
Now Bhagavan (God) keeps Bhimsen as his servant, and when he wants to give the world water he sends Bhimsen to give it. Bhimsen drags it along in the skin of a rat and as the skin rubs along over the air, it makes the roaring noise of thunder. Bhimsen rides all over the four corners of the world on his horse of lightning, dragging his rat-skin behind him.
Source:
Chapter 3, Myths of Middle India, Verrier Elwin, 1949
Kotharian Lightning 