Greentop eggs are a mutation of kiwi fruits that grow in the jungles of Ark. You can see the heart of the kiwi fruit at the center of the egg. From that fruit, the creature inside develops and is surrounded by a soft, permeable egg.
Greentop eggs can only hatch when nestled in (but not buried and covered completely) nutrient-rich, regularly moist soil.
Greentop critters initially hatch with very small wings, so they cannot fly. They can only use their legs to walk around on the ground which makes them vulnerable to ground-based predators like cats. Greentop parents, as a result, are normally pretty protective of their new hatchlings and don't let them venture far on their own.
The small plant that grows atop each Greentop's head is actually an extension of their brain, albeit a small portion. The green plant, known as the "greentopping," is primarily responsible for motion, balance, and provides an acute sense of direction. This assists them greatly during mass migrations. If the greentopping were to be severed at the base, it would render the Greentop creature motionless and confused. Greentoppings, however, can withstand small trimmings and breakage of higher-up branches. So long as the base isn't damaged, greentoppings grow back quickly.
In Ark, you don't get a "haircut," you get your greentoppings cut!