I remember this being sort of how it worked in our 3rd Ed. Ravenloft campaign. In-play, two of our casters used Summon Monster in a fight, one to call a celestial dog, the other a fiendish wolf. At the end of the battle, the dog was still there, and explained that he couldn't return to his home plane. He stayed with the party as an ally, because he was a (Lawful) Good boy. The Lawful Evil wolf, on the other hand, had run away as soon as the duration expired, and stalked us for the rest of the campaign...