Three of the bad guys sanses I've made description in comments for all three

Ragdoll: some sort of .EXE version of Sans. This Sans holds a powerful demon inside of him that only the rag on his head can keep inside the demon has fully taken over his body and mind however. You can't just take off the rag. It's embedded with some holy Magic most people cannot understand. However this does not stop Ragdoll from being one of the most powerful aus to exist even making the likes of error 404 fear. He can summon the Dead transform into a demonic Beast and even corrupt time space itself. But thankfully Ragdoll is mostly sleeping and is harmless when asleep. But be careful you never know what's hiding under that mask.

Powers: necromancy
Transformation
Reality shifting and altering
Time dilation

Delete: he once was an average sans. But like many other aus he had suffered the wrath of error. But error got creative with his torturing for delete. Instead of killing him he would constantly throw him into void like areas and screw up his body. And error gave his powers to delete the power of erasing and destroying. But yet error kept on torturing and eventually brought him to the brink of insanity his soul was in various pieces his mind didn't feel like his anymore. Inevitably error took control of delete. And delete also helps out with nightmare and his team. Delete might never find out about what he used to be like but for now he just serves.

Powers: everything that error can do.
Can transform his hands into different shapes.

..Yikes.

Ik

Ivy: once a part of ink's team of good guys Ivy has turned his back and has joined nightmare as his right-hand man. Ivy once known as toadstool was captured by Nightmare and manipulated by him. Eventually turning Toadstool the friendly guy who loves peace into with destructive chaotic former shell of himself. Ivy is much more powerful than most other aus and could easily take down nightmare himself. He chooses not to because he enjoys being on the side and not in control. Ivy has a vast range of plant abilities where he can easily control plants with his mind at will.

Powers: everything that a normal Sans can do.
Phytokinesis