The Hydra is one of the two Legendary Air Ride Machines. It takes incredibly long to charge, but it's worth the wait. The Hydra is the fastest Machine in the game and is only available in City Trial if all 3 parts are found in secret areas inside of Red Crates, like the Dragoon
The Hydra is made for speed, it can't glide very well.
It can be unlocked in Free Run (City Trial) by doing the Checklist Challenges that unlock the Hydra pieces.
With enough Charge Patches, it can charge like any other Star/Machine, but it's hard to find so many Patches. (7minsmax)