Ive never had a Halibut Fight like that, so I'm thinking it was a baby mystery beast..not an adult.
Halibut "runs" are typically pretty short lived and not the type that would ever make me fire up the main and give chase..they just shoot off in one direction for 10-30 yards, then stop.

Yep, I'm going with a baby mystery beast on this one.
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You know something bad is going to happen when you hear..."Hey, hold my beer and watch this"