"The Drake Collector"

[Deleted]

Aside from the gear thing every example you gave makes sense without needing too much handwaving to satisfy gameplay over lore:

1. The Bronze Drake we use as a mount is probably the Bronze Drake we saved.
2. Mammoths are native to Northrend so it's not all that unusual to expect a Northrend boss to have a pet Mammoth.
3. Dark Irons have mole machines so they probably stole some Kodos from the Tauren during a raid against them.

As far as I'm aware Blizzard does not consider Fey Dragons as actual dragons but Fey that happen to look like a dragon. Either way, the only way for representatives of the Dragonflights to show up is to come with us through the portal from Azeroth. The Green Dragonflight has zero interest in Alternate Draenor so why would an Alternate Draenor boss drop a mount that should only be found on Azeroth? Now if it dropped off the last boss I could see a reason as to why since last boss fight takes place on Azeroth which could have lent itself to a spore-infest Green Drake being part of the fight but that's now how it panned out.

The Bronze drake makes sense that way, in retrospect. But the other things are just random. You really have to make up stuff to have it make sense. "Maybe he stole a Kodo". Yeah maybe Garrosh brought a green drake with him.

I didn't say Fey Dragons are dragons, I meant they are native to Azeroth, so why are they on Draenor all of a sudden?
(edit: I just looked up some stuff, and saw that there are fey dragons in Blade's Edge in Outland, so maybe I'm wrong, but maybe that's because the Night Elves settled there, and they brought them with them, and the fey dragons have a connection to the Emerald Dream, didn't find anything conclusive)

You're still looking for a lore explanation, but my point was that lore really doesn't matter in what boss drops what. Malchezaar drops Gorehowl. I know that ideally a boss should drop a mount that makes sense, almost all mount drops are like that, but the green drake is only being added because it is missing from the game, so let's not take it too seriously where it could drop from.

If it has to come from Azeroth, then there is a revamped UBRS where it could drop from and it even has a proto dragon in it!