DreamMobs is a free minecraft 1.21 custom mobs plugin with built-in mobs included
Yml files --> plugin.yml/mobs.yml

Mobs.yml | holds all the example mobs and you can create your own using the template at the bottom

Plugin.yml | gives you the information of the plugin as in name version short description and permissions

Just remember this is brand new so have any bugs leave me a review and I'll fix it [:

Installation | download the plugin and upload it to a 1.21 minecraft paper server and restart then play