Stray Demon
Lore
The Stray Demon is the second boss in the Undead Asylum, but can only be fought during the return visit to the Asylum later in the game. His shape is similar to that of the Asylum Demon, but much stronger with magic explosion attacks akin to that of the Demon Firesage and Chaos Witch Quelaag
The Stray Demon can be seen at the very beginning of the game, from the hallway outside the starting cell.
He can be heard walking around underneath the floor from the Asylum Demon's boss arena. He is also likely a prisoner.
Design Works Interview
Theories
An Imprisoned High-Ranking Demon
PREV: Sif, the Great Grey Wolf
NEXT: Taurus Demon
This is just speculation, but maybe the stray demon was locked up in there. I mean obviously he cannot get out. Him being “stray” probably means he defected from the demons, and was locked up for it. I also noted that his weapon is similar to the firesage, who was deduced to be a higher ranking demon. A priest of some sort maybe. I think this shows that the stray demon was once a similar ranking firesage, defected, and was locked up. He still can use the chaos flame to an extant with his area effects, but as he is not close to the chaos flame, the flame has clearly died down in him.
Btw I got the demonfiresage= priest thing from a Dave control video