吼 means something along the lines of cry or roar. So think of it as a noise like 鳴.
- 吼(ほ)える = to bark, to bellow, to howl, to cry
- 咆吼 (ほうこう) = a howl, cry (alternative to 咆哮)
- 獅子吼 (ししく) = lion’s roar
- 帆猿 ( ほえざる) = Howler Monkey
- 吼え声 (ほえごえ) = a bark, a howl
So I guess it’s a term that’s made up in an anime or game. You said it was used in a song? It sometimes happens they coin new words in those (especially in anime and game related music). For the pronunciation you’d have to listen to the song though. If I had to make a guess, it would be てんこう.
The meaning, however, is straightforward: heavenly roar.
edited: provided more examples.