The beetle could be either a very big water boatman or a great diving beetle.
Both are carnivorous and will eat tadpoles, crustaceans and other pond life....including small fish.
If your pond is a wildlife pond, leave the beetle be as it will have its place in the food chain and will stop other species from over populating.
Most tadpoles should have now hopped off as ickle *****. Get your pond nicely established ready for the frog and toad invasion next spring.
If you don't have any fish in the pond, maybe you'd like to considder some sticklebacks. These are small, indiginous fish which have a fascinating breeding ritual. They will also help with the general bio-diversity of the pond.
Good luck