In a nutshell, the life cycle is this. The adults live during the summer/fall/winter either on land or in water (depending on their genetics + local conditions). In the spring, all adults are in water where they mate and lay eggs. The larvae grow up in water and go through metamorphosis, usually during their first spring/summer. After metamorphosis, they may stay in the water, or may move out of the water to land.
I don't think they are endangered, but whether it is legal to keep it would depend on the laws of the local country/state/town. And I agree with Chris that you should not keep it in the same tank with your frog. Alpestris are able to live on land, so it may be possible to release it to a nearby woods. It looks like an adult or near-adult, so it's probably not this year's offspring (unless it was a very fast-growing one!). So it probably came into this pond from some nearby wooded habitat.