There were no new mobs added in Minecraft 1.13.
There are no mobs in this group.
With a huge overhaul to ocean biomes many sea mobs where added, including fish, dolphins and turtles. Tropical fish can spawn with 2,700 variants, and up to 3,587 variants using commands. Drowned were added as an underwater zombie variant.
The Phantom was added as winner of the very first player mob vote in 2017.