Red Submarine (the door blocked by a dog in Junon)
WARNING: This page contains SPOILERS for Final Fantasy VII.
How do I get in the door that the dog is blocking?
First, you have to fail the submarine Huge Materia mission. Since you need a submarine to complete the game, the game conveniently has a backup sub. So, once you've failed the mission, head back to the room that was blocked by the dog. When you enter the room, you'll be able to get the Red Shinra Sub.
Does the Red Sub do anything special?
It's red. Other than that, no.