Minecraft Servers with a Pet System
Servers with a pet system are projects where the player has a companion with its own level, rarity and abilities, going far beyond vanilla tamed animals. The pet follows you, gives passive bonuses, levels up through activity and can have a rarity from common to legendary. This mechanic is popular in RPG and entertainment modes: it adds collecting, progression and character individuality. It is server content built on plugins, not a separate game.
This page lists 31 servers with pets; each project's online is verified by direct ping. Systems differ: somewhere pets give combat bonuses, somewhere they help with mining and the economy, somewhere they are purely cosmetic companions. If you enjoy character development, take a look at RPG servers too; if projects with farms and an economy interest you, see servers with an economy.