What hurt Arven's Mabosstiff? A theory

[Kirumi]

Except that the imagination theory turned out to not be the case. And Arven wasn't "very young" when it happened either.

It's however implied Arven was quite inexperienced back then, both as a trainer and human being.
I'd say this strikes to me more as a different universes' kind of stuff than an imagination related thing: we've an Universe.I. with Naranja Academy and an Universe.II. with Uva Academy.

Wait a minute, what if it was Terapagos? I wouldn’t doubt Arven’s access to the lower parts of Area Zero, being the son of Sada/Turo, and I’m sure he was curious if he ever discovered Area Zero’s lower decks. It also fits my narrative of non-version exclusive due to it happening in both versions and a Special attacker due to no physical scarring. And because of knowing Arven since he was little, and thanks to its Dark typing, I wouldn’t be surprised if Mabosstiff attacked Terapagos if Arven disturbed it, considering it a threat to Arven, and we can see Terapagos’s immense strength in the form of its Terastal state. Arven also wouldn’t have had the chance to comment on either of them upon seeing them in the game, as well.

The issue is that Arven describes it as "some kind of creature who doesn't even look like a Pokémon" and the following sentence changes based upon the version you're playing with:
Scarlet: "crazy, violent, rampaging sort of thing".
Violet: "crazy, weird, metallic, machine thing".
That's mostly the reason why I'm not assuming that we're talking about a Pokémon the Pokédex knows about, but more about something only Arven himself faced.

At the end of the day I do guess this is a clearly wanted mystery by the ninth generation itself.
Like Hassel says: "there's not a correct answer here"... The answer to this question is decided by your own soul.

 

[SlipperytheTotodile]

I own a unofficial Pokemon book that claims it was sick but I also think that one of the paradoxes injured it

 

[Blackjack Gabbiani]

Well yeah he was sick because one of them injured him. The game itself tells you this. Just not which species it was.