EastScape Wikibuilt from the game itself
Mossy egg
“Put it in a hatchery on your island with 3 Ordinary pet food: a Mossback Tortoise in 18 hours. Tradable.”
Hatches
Hatches into 
Mossback Tortoise in your island's hatchery, in 18 hours. It drops from monsters in The Gloam, The Lantern Mire.
Sold by
Nestor the Egg Man in The Yard trades it for 100 sulky sporecap + 100 button cap.
The Store: 66,000 tickets.