Eldeng eggs will materialize from Coldworld completely encased in ice that melts away quickly in Ark's much warmer temperatures. So far no one has been able to predict when or where Eldeng eggs will materialize.
When the first Eldeng appeared many thought that it was an undiscovered cousin of the Guinstar, as they both hail from Coldworld. However it didn't take long for people to realize that this was not the case. Not long after hatching Eldengs begin exhibiting signs of decay. Nothing could be done to halt or slow down the rate of decay, and it wasn't long before scientists discovered that this decay was a natural part of the Eldeng's growth and development. Whatever the Eldeng once was may never be fully known as now they are a strange combination of virus and magic inhabiting and reanimating a long dead skeleton.