Tainted Familiars are alt versions of normal familiars that act differently.
How do they appear,you ask? We'll they don't have item pools,but they do have a certain chance of replacing normal familiars,depending on how powerful they(as in Tainted familiars) are.

My Examples:
Tainted Brother Boddy:The familiar version of Montezuma's Revenge,but shoots forward!
Tainted Sister Maggy:Cannot shoot,but has a melee attack similar to Tainted Magdalene! And if this familiar kills an enemy,it will drop a half red heart lasts for a short time
Tainted Robo-Baby:The familiar version of 120 volt! He'll shoot lasers at nearby enemies
Tainted Brimstone:Cannot shoot brimstone beams,but he'll sneeze instead
Tainted Abel:The familiar version of Tainted Cain. Instead of a being a shooter,Tainted Abel will act similar to Bum Friend,she'll collect useful pickups & store them in her inventory,once Tainted Abel's got enough,she'll use them to craft a passive item!
Tainted Lil' Friend(Fiend Folio):He'll use the same fireball attack as Normal Fiend,but when he gets hit by an enemy projectile,he'll launch those fireballs,& they'll home in on enemies!

These familiars will also have sewing machine upgrades that can make them different from normal familiars' upgrades. Hope you like this idea! ;D