Tevildo

"Then the little bird sings not any more; come down or I must fetch thee, for behold, I will not encourage Elves to seek audience of me in mockery."

- Tevildo to Lúthien Tevildo was the Prince of Cats was the mightiest of Cats (possessed by an evil spirit) and a servant to Melko, the main antagonist of Beren and Lúthien.

Beren and Lúthien (2017)
"Thou smellest of dog, and what news of good came ever to a cat from a fairy that had had dealings with the dogs?"

- Tevildo to Lúthien, commenting that the latter was a friend of dogs When Beren was captured by the Orcs of Melko, he convinced the dark lord of Angband to spare him. The Ainu of Evil had him sent to the kitchen as thrall of Tevildo the Prince of Cats, who was displeased with his master sending Beren to him. So he had the Gnome serve under Miaulë the cook, who ill-treated him.

Lúthien entered Tevildo's fortress and lied to him, claiming she knew where Huan the Dog was. Tempted by the opportunity to kill his nemesis, the Prince of Cats and his thane Oikeroi left to hunt. However, Huan slew his companion and forced Tevildo to yield his collar to Lúthien and thus release the Cats from his spells. The Servant of Melko left, unwilling to either face his master's wrath or scorn.

Earlier versions of the tale
Tevildo served as a forerunner to Sauron, acting as the secondary antagonist alongside Morgoth. The Necromancer replaced the Prince of Cats and was since treated as the first appearance of Sauron, the Enemy of the Valar and Free Peoples of Middle-earth. Similar to Tevildo who is called "Servant of Melko", Sauron is sometimes mentioned as "Servant of Morgoth".

Appendices
"Tevildo - The Prince of Cats, migthiest of all cats, "possessed of an evil spirit"; a companion of Morgoth"