Merfolk

For millennia, merfolk were thought extinct on Ravnica, as the ocean was covered by the sprawling city and eventually faded from memory. The merfolk themselves only preserved dim memories of the surface, aware that there was a city ruled by guilds above them, but knowing and caring little about it. Their disinterest only ended when tree roots reached the oceans.
However, massive sinkholes enabled them to resurface, and many have joined the Simic Combine and risen to leadership positions. The new Combine has incorporated many parts of their philosophy, such as the Holdfast and the Upwelling Principles. The highest-ranking speaker for the Combine, Zegana, is a merfolk.