Looking at how the quest is worded, I suspect the chance to fuse a white flyger may only be there during the event. Not sure, but if I can get enough milk I intend going for that.
I would like to make a case for the challenges being named wrongly. The 'harder' quests are easy as they are guaranteed spawns. I struggle every day to get 5 fangbeasts. That should be recognised as the toughest.
Other than that, I'm loving this event.