Routes 210-North & 211-West, Safari Game (Night).
Source: Serebii.net
I know that on the routes the chance you will encounter a hoot hoot is still very small, 10%.
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Hoothoot is normally unavailble in firered you will need to have emerald version trade you one or you can get one from colosseum.
Yes, you can actually find Hoothoot in Pokemon Emerald. You must first beat the Elite four. As a bonus for beating it, the safari zone with have an extention. It's north of the entrance, just head up, and turn on the first right. Hoothoot are there, it just may take a while to find them.
You can find it in eterna forest (night only). I hope this helps.
you do not find dialga in Pokemon pearl
In Pokemon Diamond, I would say it is easy to find a Noctowl or Hoothoot after it becomes dark.