I'm not familiar with Tevildo either, but just looked it up on the web. According to
The Tolkien Monster Encyclopedia...
Tevildo (also known as Tifil, Tiberth) was the lord of the cats and one of Morgoth´s servants. He was black as the night and his eyes were red. Oikeroi and Umuiyan were two of the cats serving him. Morgoth had put a spell upon him and the other cats which made them huge in size and lethal in combat. Tevildo had also been given a magical collar of gold which made him even more powerful. This collar was taken from him by Huan the hound of Valinor. After that Tevildo was forced to give up the magic words, which Morgoth had used to enchant the cats, to Huan to save his life. This marked the end of the great cats. All cats, except Tevildo, were turned back to their original size and Morgoth was very angry when he learnt that the cats had betrayed him.