MiniatureMobs is a Spigot plugin that allows you to create custom mobs. You can make your own custom mobs using invisible armor stands, those mobs are able to attack players.
This resource is free from NMS and reflection.
Rotem Avitan - RDev_
https://mrgarretto.com/armorstand/ - used to design the mobs
https://haselkern.com/Minecraft-ArmorStand/ - used to rotate and fix positions
CosmeticsTreasure - https://www.spigotmc.org/resources/cosmetics-treasure-1-10-1-14-1-14-support.33313/
ZombieApocalype - https://www.spigotmc.org/resources/zombieapocalypse-premade-and-ready-server-sale.63500/
Wipeout - https://www.spigotmc.org/resources/treasurechest-2-1-10-1-14.10318/