With a +2 bonus, Summon Creature 4 correctly summoned a Summon Creature 6 dog.
However, casting Summon Creature 5 with this same boost didn't result in a Summon Creature 7 dog, but instead a Summon Creature 6 dog with a permanent duration, even persisting through rests.
The creature's name in question, both VI and VII for dog theme, is Two Headed Doom Hound.