Is this a bug? They appear like square boxes head is rolling round round.

The appearance of the Zombies is not a bug, This plugin was the first to give the option of "extra" animals and the addition of (what i believe to be) Minecraft Zombies, they only kill you through draining your health and as they make no noise can be quite a surprise when your attention is on something else (looting an airdrop for instance) I dont know what the murderers look like on this plugin as i use Botspawn.