It's a well known fact in WoW that rare non-combat pets sell for a lot. Nobody wants to farm these guys themselves due to the tedium, but they are definitely willing to pay a ton of gold to get their hands on the rarest of the bunch.
I present to you what I've done instead of my grad project:
Azure Whelpling
One of the newest non-combat pets added. It shares the same model as the other whelpling pets with an (you guessed it) azure texture.
Drops From:
Draconic Magelord
Blue Scalebane
Draconic Mageweaver
Blue Dragonspawn
All of these mobs are found in southern Azshara and have a 1/1000 chance to drop the whelpling. Their levels range from 50-54 Elite.
Crimson Whelpling
The easiest to farm whelpling is found in the eastern Wetlands. This pet tends to be the least expensive on the auction house due to its ability to be farmed by players level 25+ (Farmed the money for my first mount with one of these.)
Drops From:
Flamesnorting Whelp
Crimson Whelp
Red Whelp
These mobs range from level 24-27. As stated before, there is a 1/1000 drop chance for this pet. Note!: A common mistake is to kill Lost Whelps in addition to others while farming for this. Lost Whelps do not drop this pet and should be ignored. ("No phat lewtz? GTFO my Wetlandz!")
Emerald Whelpling
A step up from the Crimson Whelpling, this bad boy is like all other whelps except with an emerald colour. (I just Europe'd your asses!) I like to consider this one of the rarer whelps because of the lack of spawns that drop it.
Drops From:
Dreaming Whelp
These mobs are all level 35-36. They are found in southwestern Swamp of Sorrows (QQ!), a sharp right after coming into the zone from the west side. These Dragonkin shoot poison nature bolts as their main attack. There is a dissapointing amount of these fellas around, so if you're farming rate is quick enough you may have to waste time waiting for more to spawn. Be careful not to agro nearby cheetahs or spiders, as they are a waste of time.
Dark Whelpling
The second most profitable whelp, in my opinion. It's easier to farm than the Emerald ones, and the zones it can be farmed in are accessible by less lower level players.
Drops From:
Scalding Whelp (Badlands)
Searing Whelp (Dustwallow Marsh)
These dragonkin are level 41-43. I'll begin by highly recommending Badlands over Dustwallow for this farm. In Badlands the area you are farming in eastern Badlands is only the subject of 1 or 2 quests, and is filled almost exclusively with Scalding Whelps. Dustwallow Marsh recently became a leveling hotspot due to the addition of Mudsprockets, so there will be low level competition for these mobs and they share room with other useless dragonkin.
Netherwhelp
Oops! You're shit out of luck, son!
Drops From:
Burning Crusade Collector's Edition
Unless one of these bad boys is floating around on eBay there's no way to get your hands on one. Sorry!
Enough with Whelps! Let us delve into the lesser known world of:
Disgusting Oozeling
You probably saw more of these suckas floating around before TBC. I know I did.
Reason being?
Drops From:
Oozing Bag, which drops from: A variety of Oozes level 50-60.
To get one of these pets you first need to get an Oozing Bag. These have about a 5% drop chance from Slimes and Oozes. After obtaining one of these bags you only have a 1.2% chance of getting your pet. My personal favorite place to farm these slimes is inside the various scars of Eastern Plaguelands. Un'Goro Crater also has a decent slime count. Note!: This pet will turn you a snot-green while you have it out, and decreases your defense and resistances by 20.
Captured Firefly
This pet can only be farmed with a copy of The Burning Crusade. It auctions for around 500 gold to 1000. (Of course there will always be idiots that you can sell this to for more).
Drops From:
Bogflare Needler
These wasps are level 62 and can be found in Zangarmarsh. There isn't much more assistance I can give you as to finding these guys. Note!: Spore Bats that spawn near your targets will give a very nice buff for killing them. This could help increase your farming rate. Good News!: Farming these bugs will net you greens and grays that will make you a profit. Time spent farming this pet is not wasted time.
Hyacinth Macaw
Back to Azeroth! Quite possibly THE rarest non-combat pet evar! These birds have a drop chance of 1 in 5,000. (Regrettably NOT over nine-thousand.)
Drops From:
Bloodsail Swashbuckler
Bloodsail Mage
Bloodsail Warlock
Bloodsail Deckhand
These mobs range in level from 40-44. They can all be found in the coasts and ships surrounding Booty Bay. Note!: Only a true pet connoisseur will pay top dollar for this beauty. Most noobs will value this as just a common bird like that piece of shit that drops in the Deadmines. (Pardon my French [You probably don't care.]). Keep at it and don't dip too low in price and you can probably find someone who truly appreciates this pet.
Lifelike Mechanical Toad
Name says it all. (Not really.) This pet doesn't actually look mechanical at all, which is somewhat dissapointing.
Drops From:
World Drop (46-60)
Yup. That's right. Those two suicide-inducing words. (Maybe I play too much WoW..) [Maybe I write too many of these stupid notes..] Anyway. This toad, despite being pretty much impossible to farm, nets very little on the House of Auctioning. If you're farming another rare drop in Azeroth don't pass up this little guy.
Thus my profitable pet guide is to be concluded! It's been a wonderous journey and I've already forgotten what other cheesy motivational phrase I was going to slap onto the end of this sentence. I did not include BoP rare pets, and those I deemed not rare enough / not expensive enough. This is my first guide ever and I apologize if this topic is unimportant and irrelevant to many. Hopefully this is not a repost as I searched the board before I thought to do a guide on this topic and I found nothing.
Best of luck in your pursuits!