Ahh okay. I played warspear which has a mechanic like that. Basically if you werent in the party that had agro it wouldnt even count that you killed it or helped kill it.
But the drops were random for every player in a party sometimes people would get something most the time they wouldnt. How it worked was who ever got the drop kept it.
Yes, Celtic Heroes is along those lines as well. There is a lock mechanic, where the first group of players to do a certain amount of damage will "lock" the boss or mob, locking the rewards (any gold, exp, or drops) to that group only. If all the players in the group die at once, the lock is reset and another group has the chance to take the lock.
If you need to kill a boss or mob for a quest, you need to be in the group that locks the mob, otherwise you won't get the kill credit.
Note: the lock mechanic is not directly tied to the aggro mechanic. The general boss-killing process is to have 1 group with a tank and several healers that can hold the boss's aggro, and another group full of high damage players that can focus on dealing damage and getting the lock for the clan.
The drops are randomly distributed by the game in the group that kills the boss, yes. However, to be fair to all players involved, (because most bosses require more than 8 people to defeat them), different clans across the servers have their own ways of dealing with drops.
I believe that whenever a group of players (more than 8) kills a boss in Epona, they all get together and roll for whoever gets it. If there are less than 8 people at the fight (and they are all grouped together), then usually whoever gets it just keeps it, since it was randomly passed out anyways.
Other clans and servers might collect all boss loot at the end of the fight, then list them out in an external chat app or database for players to request based on participation and need.
Some clans and servers (like mine on Sulis) does a bit of a hybrid between the two, where weaker drops are rolled for, but the better drops need to be requested.