In Monster Hunter Online, you fight Plesioth at Great Fortress Lake by riding on special Wyvern Hunting Boats. This event is only available for a very limited time.
The various supply drops that you'll see are used to either refill ammo or give various buffs, depending on the flag color.
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And here's the Plesioth boat battle... Video 14:42 Kogath - Monster Hunter Online - Plesioth...