Bro, Shadowlurk as an egg is looted from Elite Caves of Tarewa, I get it once there.
Any pet that you bought from stable will always as an pet (automatically). That's as far as I know.
And if you want to buy as an egg, you could buy from NPC, another player, and auction house.

At some event, yes there is NPC who sell egg such as mythic egg, it happens on halloween event last year.
So why bought something that you can get by farming elite or in auction.

Just give my opinion, save your plat for new mythic event, enjoy the game ^^