Blizzard has posted some information on the drop chance of Tyrael’s Charger and the 36-Slot bag from the Diablo x WoW Crossover event.
Just a quick callout on two of the items available during this event. The Tyrael’s Charger mount and the Horadric Haversack 36-slot bag each have a significantly higher chance of dropping on the first Treasure Goblin kill of the day on any character on an account. Subsequent kills or kills on alts have reduced chances for these items.