A Hydra is a mutli-headed mythological creature.
Somewhere in the galaxy, a Hydra-like creature exists. The Skrulls were able to record this creature and could project a hologram of it. They used such a projection on Earth at the Bay of Fundy to distract the Fantastic Four.
The Hydra is not actually named.
In the Comics
The villain team HYDRA takes their name and logo from this creature.