Spawning in a pet human, it spawns as a zombie with the nametag human pet. Toggling it with pet selector, crashes the client and everyone nearby. The icon of human pet in the selector is a stone block.

Jenkins build 155.

EchoPet only supports CraftBukkit builds, not unofficial Spigot protocol hacks. This only occurs in 1.7 because the client can no longer receive the packets the server is sending to show the Human Pet.