Giant Mushroom Mob Generator -- With a couple stacks of dirt and bonemeal, and about 20 minutes, you can build your own high-throughput mob generator [Video in Comments] : r/Minecraft
MINECRAFT 1.8 Custom Mob Generator! (Tutorial)
I tried using the summon mob generator. It worked but the zombies are not moving : r/gamergeeksnz
Summon Bat Generator, Minecraft : Gamer Geeks
Broken Spawner/Generator ! | CubeCraft Games
Minecraft Summon Mob Generator (Java Edition)
Ritter Ultimate Event Spawner (RPG Maker MZ) by notRitter