Pokemon fusion maybe? Never played it though but that one should be by nature.

Okay there are several I can think of.
First and probably the closest to having minions who are customisable characters is flexible survival.
You can have mutant children in the game, provided of course you get pregnant (given the game's theme something that will happen). Your children are essential deployed as combat items, they can learn abilities and be set to different roles, and can level up and gain experience, and there are perks you can take directly connected to your offspring, EG perks that reduce the lag time in your own attacks caused by your children. Indeed, Flexible survival has one of the most unique and complex battle systems of any mud, and the children are just another aspect of that.

First thing of course, Flexible survival's theme is pretty adult, being an apocalyptic world full of sex crazed feral mutants, and though the mud  on the hole less extreme in its descriptions than the single player game, don't try it if you've got an issue with adult material.

Also, bare in mind to have children you need to get pregnant, which can occur with another pc or a monster or npc, however, pregnancies progress in real time days, so don't expect to get kids the second you start.

A more traditional option is erionmud. This has  familiars which work like extra npcs and fight along side you. Familiars grow, need healing, level up and can take damage, though only some classes have them and the maximum you can have is three, and that only by taking the archer's beastmaster subclass.

I also believe cosmic rage has drones which can do various things throughout the game, but that is a mud I need to check back to since its changed quite a deal I think since I last played.