@Kamboozle Squirky is not yet available, you'll know it when it happens!— Jeremy Feasel (@Muffinus) April 12, 2017
This is a little surprising as he has hinted at many "secrets" still undiscovered in-game. Many collectors assumed Squirky was on that list, but it would appear that we're mistaken!
So where could this pet come from? Wowhead Site Director Perculia speculates that it might be from a promotion (either BlizzCon or other), due to the fact that one of the advertised plushies is a yellow murloc.
Personally, my hope is that it's not tied to a Blizzard Store item and that Squirky will be readily available in-game. Collectors have waited this long, and still continue to patiently wait, so it would be heartbreaking to find out the availability of this pet is limited only to those that can afford to buy a physical item.
In the end we'll just have to wait and see!