Well, I have come across the exact same situation, in my current gaming run, with a small difference: the Archon was confused.
Now, the main difference here is confusion. You didn't specify whether the attacks continued after your stethoscope results came negative, so it is plausible that on the turn which you checked your pet's status, the confusion wore off. Quoting from the Confusion wiki page:
Monsters have a 1/50 chance of losing the monster confusion status on their turn. If monsters have access to a non-cursed unicorn horn or a lizard corpse they will use it to clear the monster confusion status.
Also on that page there are the possible reasons a creature might become confused. The only possible ones are the following:
- Being affected by vapors of a potion of confusion or potion of booze.
- Being exposed to an umber hulk's gaze attack (I place this here because I didn't remember about this question until it happened, and the Archon killed an umber hulk, whose body I found after).
- Monsters which are clerical spellcasters can confuse players without magic resistance. (this can be ruled out as there were no spellcasters nearby).
In my game, most creatures at first sight either drink whatever potions they have or they throw them at me. Although most of them were paralysis, I believe if they had potions of confusion, they would throw them either at me or at my pet.
Also, after checking the wiki, I found out that the umber hulk's gaze attack isn't resistible (as it isn't a magic effect).
After I noticed, I began checking the status of the pet each turn (as possible, since it is a very fast pet) and after 27 turns passed, the pet regained his composure. I have a certainty that he didn't have an unicorn horn (because I polypiled those that I've found at Orcus' Town) or a lizard corpse (because I didn't find one in Gehennom).
My explanation is that either the RNG God had pity and decided to cure your pet at the precise turn you would be able to use a stethoscope on him or the pet had one of the items that instantaneously cure confusion.