"Whoever fights monsters should see to it that in the process he does not become a monster. And when you look long into an abyss, the abyss also looks into you." (Beyond Good and Evil, 146)
The monster is a monster largely because of the lengths it will go to in order to defeat or harm the good.
This risks leading the good into a position in which it must fight on the same terms in order to have a chance of winning (or merely surviving).
When this happens, it can become more and more difficult to distinguish the efforts of the good from the efforts of the monster.
Take the example of an enemy who is willing to kill. When faced with such an enemy, a good person may be drawn into a situation in which they must acknowledge the need to kill in return (or to kill pre-emptively), lest they be defeated.
Perhaps this is what Nietzsche meant when he said:
when you look long into an abyss, the abyss also looks into you
Did he mean that when you look into the nature of the monster who is willing to destroy, that the willingness to destroy finds itself within you? Was he pointing out that the person who takes a knife to a gun fight is likely doomed, and so the threat of a gun (the monster) necessitates the threat and possible use of a gun (or something stronger) in response? Was he pointing out that the use of a gun, ordinarily deemed monstrous by the good, often becomes necessary upon confronting the monster for whom the use of a gun is mundane?
I can't provide a definitive answer as to Nietzsche's intent, but this seems a reasonable interpretation to draw in the absence of any material describing his motives for the quote.
The 'abyss' speaks of a psychological state; an awareness, upon using the gun, that the monster lies not only in front of you, but within you. So, when you ask for 'examples of a monster', you are limited perhaps only by the strength of your imagination, for monsters are many and varied. The monsters of your own psyche may well be lying in wait for you to stumble upon them, perhaps during some attempt to be the vanquishing, vengeful hero, perhaps merely in your unexamined desire to be such a person.
Nietzsche talks about the dragon 'Thou Shalt', as the opponent that requires a lion-nature to face in his metamorphoses. Discussed here, along with the Monster Theory interpretation if what Nietzsche was up to:
As a friend of Wagner there can be no doubt Nietzsche knew about the dragon Fafnir, who had been a human until slaying his father to take the cursed ring Andvaranaut which could be used to find gold, and became a dragon to guard this and the hoard taken from his father.
As a philologist he would undoubtedly have been familiar with Python, who was slain by Apollo and buried in a cave under the Omphalos marking the centre of the world, and who's fumes are said to have inspired the Pythia, the Oracle at Delphi.