Sun Darter Hatchling
Drops from Oddly-Colored Egg. This unique species of faerie dragon is not usually found on Azeroth. Dragonkin type. Check my comment for a guide on how to obtain this cute pet.
The 
Sun Darter Hatchling is a secret pet obtained by solving a puzzle-based riddle challenge, ultimately rewarding you with an Oddly-Colored Egg found inside a cave north of Winterspring on the coast of the Great Sea.
Guide awaiting review
Gather Protection Potions
Before approaching the cave, collect one each of 
Major Arcane Protection Potion, 
Major Holy Protection Potion, 
Major Shadow Protection Potion, 
Major Fire Protection Potion, 
Major Frost Protection Potion, and 
Major Nature Protection Potion. These can be bought off the Auction House or crafted by an alchemist with skill 360.
Obtain Alchemy Recipes If Crafting
If you plan to craft the potions yourself, the 
Recipe: Major Arcane Protection Potion drops from Vir'aani Arcanist around Oshu'gun in Nagrand (Outland), and the Recipe: Major Holy Protection Potion drops from Abyssal Flamebringer. Note that these recipes only drop for players with the Alchemy profession.
Collect Remaining Required Items
Purchase 20x 
Noggenfogger Elixir from Sprinkle Noggenfogger in Tanaris, obtain 
Scotty's Lucky Coin by defeating Scotty in Townlong Steppes, and acquire 
Dire Brew by speaking to the Dark Iron Brewer in Blackrock Depths.
Locate the Cave Entrance
The cave entrance is not inside Winterspring itself — fly due north from around 48, 1 in Winterspring toward the coast of the Great Sea, where you will find an area of trees and stones concealing the cave entrance.
Navigate the Cave Carefully
Once inside the final room after receiving the intellect buff, do NOT jump down into the water — doing so will teleport you outside and likely remove your one-hour buffs. Instead, move all the way to the right and enter the water from the slope to safely reach the Oddly-Colored Egg.
Tips
The protection potion materials are frequently bought out and overpriced on the Auction House — consider farming or crafting them yourself to avoid inflated costs.
This pet's acquisition method is similar to Kosumoth the Hungering and
Riddler's Mind-Worm, so the WoW Secret Finding Discord is a useful resource for additional puzzle guidance.See the video guide for a full visual walkthrough of the cave and all required steps.
