Well here’s a nice little surprise for players, as there are many reporting receiving a store mount they not only did not buy, but one that hasn’t been available for years!

A Mists of Pandaria mount, the Swift Windsteed was retired from the Blizzard store back in January 2019, and today a number of players have reported just receiving it in their mount collection as a gift box, despite never having bought it (even while it was available) – and yes, it is a flying mount.

No idea. I got the mount too – KyleEverett

Same. Freaked me out a little bit. – Cruelsader

This happened to me too! Not the cenarion hatchling though, just the swift windsteed – treebalm

I just got one too. Asked my spouse to check hers and she did not get one. I don’t mind the free mount but what is going on? – Kikjin

This following player also received a Cenarion Hatchling in addition to the mount, on a different character, but they are the only one that reported getting two things.

The official forums are also full of players reporting receiving the mount, as you can see in this thread:
Just got a free Swift Windsteed?

So, maybe check your mount journal if you were one of the lucky ones randomly receiving an old store mount and let us know!