There were high Level Monsters between LvL 5 and LvL 15 areas.
In Gridania the wolves (problem there was the small pathing)
In Limsa the Gobbue (if you see something big, run around and avoid it)
In Ulda i think there were dracons roaming (compared to limsa, there were more of them, harder to avoid)
Hard to remember because they patched them out soon after release because many dead players in starting areas (but later on in endgame areas the LvL99 monsters were ok?)
Players complained that they being oneshotted even when they follow the path (just like in the wizard of oz: dont leave the path!)
It was a good gamedesign with those roaming "dangers" but... in Gridania it was pure pain because of those small paths!
However, i agree: they needed a better AI because with bad RNG they were farming low level players...
Here some rare videos - Gobbue shows it best:
Escape impossible - he can throw stones!
Dont provoke something that looks stronger - he might get angry!
Just to show he was beatable later on - or to watch nostalgic combat system!
Best thing on open world was: You just asked in area chat for help and players were helping - since 2.0 instance only dungeons you can not do that any more!
We have beaten unbeatable LvL99 Monsters because the whole server helped: With about 60 Players you could count 2 hours, the more the faster it went down!
Try to go into a 2.x dungeon with more than 4, 8 or 24 players - its not possible!
Edit: We had sight aggro: run or walk behind them to avoid,
hear aggro: you could walk to avoid them,
smell aggro: go into close range and they will follow you,
none aggro: if you dont attack, they will not too.
