It is not known to evolve into or from any Pokémon.
Seamapan is a Luvdisc-like Pokemon with a striped pattern resembling a pair of panties (Shimapan). Its dorsal and anal fins are positioned to look like a pair of legs attached to the “buttocks” formed by Seamapan’s peach shape.
Seamapan closely resembles a piece of female underwear designed to resemble a fish. It bears a resemblance to the Pokémon Luvdisc. It's shiny could be based on Marge Simpson from The Simpsons, as she is fucking everywhere on rule 34.
Seamapan is a combination of sea, referring to its aquatic nature and shimapan, the Japanese term for striped panties, which Seamapan's body is modeled after. It may also be a play on “see my pan(ties)”.