Xenopus laevis
The leptin (LEP) hormone was long thought to be specific to mammals, but in recent years the gene ("lep") has been found in amphibia such as the tiger salamander ("Ambystoma tigrinum"), and the African clawed frog ("Xenopus laevi"). The discovery of "lep" in puffer fish ("Takifugu rubripes") demonstrates the ancient ancestry of this hormone.