A Thestral, the first of his herd to be born in the Forbidden Forest and a particular favorite of Hagrid (OP21).
When Hagrid called the Thestrals during a Care of Magical Creatures lessons during Harry's fifth year, Tenebrus was the first to arrive (OP21).
Tenebrus is a member of the Thestral Herd living in the Forbidden Forest. When the herd came to the Forbidden Forest, it consisted of one male and five females (Pm); Tenebrus was the first Thestral born in the herd's new home (OP21).
From the word "tenebrous," which can mean "murky"/"obscure" in the sense of being blocked from light and thus dark or in the sense of being hard to understand. It can also mean "gloomy." From the Latin tenebrae, meaning "darkness" (source: Merriam-Webster).