The frogs don't come from another dimension you know. They live on a mountain. It's a real place. The summoning jutsu just teleports them to the user.
The same could be done with any animal.
As far as the pig is concerned, Akamaru just barks also, but he's clearly intelligent and capable of performing jutsu. Yet Kiba's mom's dog can totally talk.
I think the difference between normal animals and ninja animals is really just a matter of training.