Source: WowAce

I doubt it can be bonus rolled, and not sure if the drop rate has changed? Otherwise this should be fairly easy.

From a bluepost:

There has been some confusion over how this drop works so we wanted to provide some details.

The mount has a 20% drop chance multiplied by the number of loot eligible players (even if they have the mount).
Note: The mount will only drop for someone who doesn’t have the mount.

For example:
5 players who are not loot locked = there is a 100% chance for the mount to drop
4 players who are not loot locked + 1 player who is loot locked = 80% drop chance.
4 players who are not loot locked & have Nightbane mount + 1 not loot locked player without Nightbane mount = 100% chance the player without the Nightbane mount gets the mount
4 players who are not loot locked & have Nightbane mount + 1 loot locked player without nightbane mount = 0% chance to get Nightbane mount (the only person who could get it is already loot locked)
4 loot locked players + 1 not loot locked player, none of whom have the mount = 20% drop chance. The mount would only go to the non-loot locked player."