Moltres ran on turn zero wth

[Tapil]

saved all my masterballs for whatever legendary roaming poke there is in this game (Y) and it ran on turn zero? wth!

[icycatelf]

You can't catch it until it roosts at the Sea Spirit's Den. I think you have to encounter it a few times before it goes there.

[Rin Kairiu]

Tapil

saved all my masterballs for whatever legendary roaming poke there is in this game (Y) and it ran on turn zero? wth!

You have to encounter it ten times as it's roaming before it goes to the Sea Spirit's den where you can catch it. You can look at your map to see where it is currently, but don't fly there, because it will always move if you do.