It looks like a greengill to me.
You can legally re-release it into a private pond as long as the owners don't mind and the pond is not connected or doesn't drain into any local waterways (such as creeks or rivers).
Private pond would be considered an isolated body of water, manmade, that is on private property and not owned by the local government or state.
Even a naturally occuring isolated pond might be considered legally ok to release into, but I wouldn't want to potentially harm the natural ecosystem.