To the point: Why do the Bronze Whelpling companion and Green Drake mount not exist yet?
The simple answer from Blizzard would probably be: "We have not yet found a place we feel appropriate to introduce them yet."
So here are my ideas:
1. Zone Drop: Tanaris; 1 in 1000 drop chance; Why? Caverns of Time open there. So either one wandered out and got picked up, or in a zone full of treasure hunters in one form or another maybe someone snuck in and stole one. (Makes it on par with other whelp pets)
2. Dungeon Drop: 1 in 1000 drop chance off any Infinite Dragon mob in any CoT dungeon.
3. Boss Drop: 1 in 100 drop chance off any or a specific Infinite dragon boss.
4. Quest reward: In Tanaris, starts with a gnome in a yellow robe asking your help in reacquiring a stolen artifact. During the chain you discover the artifact is a dragon egg of the Bronze flight. Last part in chain involves an Infinite Dragonkin trying to corrupt the egg. After you defeat him you learn the egg was meant to find you the whole time and the Gnome who gave you the quest is a grown up version of the whelp to be born, who needs to now spend some time under your protection. (your companion) (Inspired by Capture Flamefly quests and Chromie)
Green Drake (I don't have as many ideas for this, I collect pets mostly)
1. Reputation: Exalted with Guardians of Hyjal faction, or other druid related faction.
2. Achievement reward: Emerald Dreamwalker, Reach Exalted reputation with the following factions. Cenarion Circle, Cenarion Expedition, Guardians of Hyjal
3. Rare Spawn drop: Add a new corrupted green dragon spawn at one of the four emerald dream portals. I know the nightmare has been contained to one little area in the dream but lets call this guy a fluke who escaped and is trying to get back in. You defeat him and...win him? Soloable, but a challenge in some way (a side from being in one of four different zones when he spawns)
So those are my thoughts for the only missing core color pet/mounts left to be introduced. Thoughts?
(I posted this on the WoW forums a few days ago but wanted to see what folks here thought as well)