Tenebrus was a Thestral who lived in the Forbidden Forest. Part of the Hogwarts Thestral Herd, he was the first Thestral to have been born in the Forest, presumably to the male and one of the five females initially acquired. He was the personal favourite of gamekeeper Rubeus Hagrid. In 1995, when Hagrid introduced his Fifth year Care of Magical Creatures students to Thestrals, Tenebrus was the first to appear after he called for the herd.
The name Tenebrus is presumably derived from the English adjective tenebrous, which means "dark," "gloomy," or "obscure".  This word is itself derived from the Latin tenebrae, meaning "darkness". 
- Harry Potter and the Order of the Phoenix (First appearance)
- Harry Potter and the Order of the Phoenix (film) (Possible appearance)