The Wind Rider Cub was recently back in the Blizzard Pet Store, however it seems that the sale was short-lived. After roughly a little over a week and a half the store sold out and now the option to even preview the item/pet in the US store is gone.
The EU store is also missing the listing for the item/pet, but then again there's a notice on the front page that physical items are currently unavailable from that region's store (this includes both the Wind Rider Cub and Gryphon Hatchling).
When the little wyvern sold out in the past, players were still able to at least view its Pet Store page. Now that it's gone completely, everyone is wondering if this means it will be gone for good.
It's entirely possible that this is only temporary and Blizzard has removed the Wind Rider Cub's Pet Store listing to avoid any confusion about whether or not it's available. Perhaps it is simply out of stock and it may take some time before they're able to create more. We'll just have to wait and see what's planned for the future. Hopefully, for all collectors, the Wind Rider Cub will be back in some form or another.
Unlike its counterpart, the Gryphon Hatchling (who shares its model with the capturable Wildhammer Gryphon Hatchling), the Wind Rider Cub has a unique model and does not share it with any other pet. Because of how unique and rare it is, it's in somewhat high demand by collectors. It's adorableness and limited availability also drives up the desire to own a little wyvern companion.
If (and I stress the IF) the pet is truly sold out from the Blizzard Pet Store for good, one thing that can be done to compensate is to add a wild version of the Wind Rider Cub into the game. This would allow players to collect at least some version of it, even if they can't get a hold of the store pet. It's not 100% the same thing, and some collectors will still pine after the "original" one, but there are others who will happily seek out and capture a wild one just the same.
So while it's not time to worry too much just yet, we'll definitely need to keep an eye on the Blizzard Pet Store and hope that the Wind Rider Cub will return.