From Destinypedia, the Destiny wiki
The Pouka is a small airborne fish-like entity that accompanies Elisabeth Bray. Judging from her interactions with it, it does not appear to speak, only make simple cries.. The nanotechnology contained within Quicksilver Storm directly identifies the pouka by its species, specifically it being a juvenile, so this may hint towards pouka being a native species or creation of the inhabitants of Neptune thanks to this foreknowledge. It is first encountered by Elisabeth on an assumed-abandoned space station, far out in space towards Pluto, accompanied by the Ghost Tokki.
As to its activities in its partnership with Elisabeth, it appears to have performed reconnaissance for her, as Eris Morn spotted it hanging around her camp during Season of Arrivals, mistaking it for a Ghost. Drifter, as with all things, appears to distrust the small being, as he and Eris discovered it alone at the Beyond camp on Europa. Even though Eris and Drifter have been working with Elisabeth for some time now, not even they have been given any information as to know more about it.
For now, the pouka's origins, purpose, and future remain a complete mystery.