To the point: Why does the Bronze Whelpling companion 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)
So those are my thoughts for the only missing core color pets left to be introduced. Thoughts?
The model exists already so it shouldn't be hard.