how do you find Ny'vene egg

> Buy them from the Arena in Ethernia for a ludicrous amount of gold (or hatchlings, for arena marks), or
> manage to beat Vierna (boss of the Dungeon of the Forsaken) in under a certain number of moves (I believe it was estimated to be under 25?) and then get really lucky with her loot drop table RNG and have it roll you a Forsaken Guardian vene
> make them during a fest in the garden/sandbox/brewing/sculpting or win them from the boards or obtain as some other fest prize (eg. theme design getting picked, making it to final door in pick-a-door, etc)

you cannot find a ny'vene egg in the wild as you would a regular species.

To add to Scathreoite's post :

The comunity garden of the spring festival enable you to encounter adult ny'vern in the wild when that speacie's flower is in bloom. This is, so far, the only way to actualy catch a ny'vern in the wild. ^-^