I checked the wiki, and there is no such pet recorded on there, this is a mod(Calamitys addons?) or just some fake screenshot.
Calamity BABY AstraEGGdon is a craftable and an extremely hidden pet summoning item, which can also be dropped extremely rarely by Astral Slimes in Death Mode, or guaranteed from Supreme Calamitas in any mode if the player takes no damage during the fight. Taking damage restricts the drop to...
I've made the mod, created the screenshot and also created the wiki page. Oh, I've also modified the code in all of your games for it to appear. x'D