The Umisen-Yamasen (海千山千) are a type of Mushi which are sub-categorized by where they live. Both look like small, snake-like creatures but one lives in forested areas while the other lives in the ocean. After one thousand years both of the Mushi come together to combine to make a single Mushi. The ones living in the sea can create a mist that anyone can see out of so long as they want to get back to shore.
This Mushi's name comes from the Japanese expression 海千山千 (umisenyamasen), literally meaning "ocean-thousand, mountain-thousand". This expression is used to refer to a person who has had enough life experience to get out of any situation using their wit. It refers to an old saying that a snake must live one thousand years in the ocean and another thousand years in the mountains before becoming a dragon.