Gama sennin with his toad
Ivory sashi netsuke, approx. 10cm in height. Victoria & Albert Museum
From Michael Ashkenazi's Handbook of Japanese Mythology:
"Toad sage," also known as konsensei. An elderly man with a warty hairless skin, he is said to live forever, able to change himself into a toad. He is one of the immortal sages, of Chinese Daoist origin, who are associated with longevity and medicine. Gama-sennin's constant companion is a three-legged toad, whom he often carries on his hand or back. He is also the creator and manufacturer of magical pills, and whomsoever manages to secure them is able to extend his lifetime. Gama-sennin himself sheds his skin like a toad, and thus renews himself as necessary.