I read somewhere the one of the neutral ending had the annoying dog as leader, it's truth? How it happens? (Not that I want to have it, I'm too pacifist XD)

(undertale spoilers)

Yes, this is true. If only Toriel, Papyrus, Undyne, and Mettaton are killed, the Annoying Dog takes on the duty as leader of the underground. No other monster can be killed, meaning the kill count must be four.

This ending is by far the strangest. The Dog Song cheerfully plays in the background as Sans explains that a dog (presumably the Annoying Dog) has become president after the other leader candidates have disappeared. Unlike every other ending in which Papyrus is dead, Sans does not mention his brother nor shows any hostility. It’s rather uncharacteristic of Sans, but this is a canon ending to the game.