Just started watching TFS DBXV2 play-through in an insomniac binge.

Jaws the Shark, Rexor Tyrannis and Nosey the Ferret

Criminals for hire, with Jaws being the leader. The main basis for Jaws and his two cronies was having them being a parody of 80s cartoon villains. Namely, The Shredder and Bebop & Rocksteady. Including Jaws' voice being similar to the late James Avery.
Originally, Jaws lacked any attire, so this is a recent update of him. Rexor is also given feathers based on current research.

Lord Squidflayer and Bruto the Nautilus

An evil squid wizard and his bodyguard. He resides on an island populated by monsters inspired by Ray Harryhausen films.
He often fights with Knuckles for the Master Emerald, but he will try to get his hands on the Chaos Emeralds as well.
The designs are by me. Having tentacles on their overbites with teeth ending on beak-like fangs. The nautilus base is inspired by the armadillos like Mighty, only with a striped pattern. Both have tentacles forming tails they can also use as weapons.

Professor Herbert Rattus and Dr. Issac Venom

A mad scientist rat and his salamander lab assistant. Usually posing as a threat to Tails most of the time. Rattus is obsessed with genetic tampering, alongside body horror surgery. As you can see, his left hand and right side of his face are deformed and a drill attached to his tail. Venom has a metal plate on his chest protecting his artificial heart and dons a scar on his eyebrow.
Like Jaws, I updated their designs from my old ones I have archived in my binders and books. They even had more boring names back then, being "Professor Labrat" and "Igor". Instead of a deformed arm, Rattus originally had a cybernetic clamp claw.
(Ignore the typo on Venom's name ) 

The D.N.A. Monster

One of Rattus's prized creations. Made at the end of a DooM crossover I wanted to make, with Tails teaming up with DoomGuy, fighting an invasion from hell on Sonic's homeworld. Rattus preserved a corpse of a Hell Knight and mixed its DNA with a blood sample of Sonic he managed to get, resulting to making an abomination of a creature. 
It is still sentient, deciding to go on its own motivation, becoming an arch rival for Sonic, as it has his speed alongside the Hell Knight's brute strength. Its design is based on the 2016 reboot's Hell Knight. And yes, the eyes are pentagrams.

Jakamarra the Echidna Lich

Dwelling in the depths of the ancient echidna ruins the tomb of a corrupted echidna shaman. He was executed by Pachacamac long before Chaos existed. Rising from his grave, he becomes a menace for Knuckles with his army of undead echidna warriors and monsters associated with him. All that remains of his body are his decayed head and bone torso.
His hands and feet float, ala Rayman, and wears a torn cape. His hands lack the spurs the males have as they were removed from them as part of his execution. If you've seen my Echidna Ruins tileset, this was the badguy I planned for it.
And here's a fun fact: I got the name from an Aboriginal name list. Since echidnas are Australian animals, it felt appropriate.