Yarayaho eggs aren't really eggs but a larval form of the Yarayaho. They attach themselves to other creatures, like a tick or leech, and will feed on the creature's blood until it falls off and grows until it can move around on its own.
Yarayahos are said to ambush solitary travelers in Ark's forests, leaping down upon the unsuspecting victim from trees or out of dense bushes. The Yarayaho will then suck the blood of their victim leaving behind abnormally large bite marks. It is said that if someone dies from a Yarayaho's bite they will turn into a larval Yarayaho.
Most people don't believe that Yarayahos exist as many of the reported Yarayaho attacks can be explained away as the work of other creatures, accidents, or deliberately falsified. However there are a small number that can not be explained away and appear genuine.