Puppies!

 is a perk added to Fallout 3 with the Broken Steel add-on.

With this perk Dogmeat is reincarnated as a puppy every time he dies and can be added back to your group either by finding him resurrected at the location of his death or by waiting until he is moved to the Vault 101 entrance. Dogmeat's puppy is much tougher than the original Dogmeat. Dogmeat's Puppy will level with the player in terms of damage done and health.

Despite the name, this perk only allows you to get one Dogmeat's Puppy at a time. If it dies, it will be replaced by a new puppy.

This perk still works if Dogmeat died pre-Broken Steel.