How Do You Make an Entity Attached to the Player?

I asked about this a while back, so hopefully second time's a charm.

I was wondering how you can make an entity that gets attached to the player, kind of like how armor or parrots work; for example, if I was making a mod that centered around disease or something and I wanted the player to grow bumps, or if I wanteto let the player have a tail.  Something similar to what I am talking about would be the Tails Mod for 1.12.2.

If anyone has any tips on how to do this or questions, don't hesitate to respond.  Thanks!