I mis-remembered. It's a monster from Rogue! I could swear I played some Rogue variant very long ago where it was spelled with an "a". Quick research shows that Rogue had it as "kestrel" since forever, though. These "hallucinatory monsters" ("bogusmon") seem like a new addition to Nethack.
Heh, I wondered if you were actually remembering such an esoteric fact about NetHack.
Looks like kestrel in NetHack dates back to version 3.1.0, released January 1993. The comment says "Rogue", so I'm sure it was done in tribute. They also added emu, xeroc, and venus flytrap as monster names from Rogue, used while the player is hallucinating.
