Spanish to English Art/Literary Linguistics

-Aceptemos por tanto que la mayoría de la gente que está ahí fuera exija nuestras cabezas, o sea que cualquiera de ellos puede ser ese tal XXX que por muy ridículo que suene el ***** nombrecito ***** resulta evidente que matar, mata en serio. Buscarle sería como buscar una aguja en mil millones de pajares.

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This seemed too obvious at first, but "little name" just wouldn't do, whichever way I looked at it! It appears all over the place on Google, but seems to mean different things in different contexts - but so far I haven't found one that fits this one.

CONTEXT: the people speaking here have just received a very threatening letter from XXX requesting certain actions, OR HEADS WILL ROLL!

The letter is signed "XXX", which is also the name of a famous fictional character.

My attempt so far is "assumed name" - since this in fact what it is - at least, it's presumed to be an assumed name. I don't think "nickname" works, as it's a name this person has supposedly chosen for him/herself. Or is there another meaning that I haven't thought of - slang perhaps?

Strangely enough, despite being rife on the internet, nombrecito doest't appear in RAE, Pequeño Larousse, or my trusty Collins.

Any help would be most welcome - many thanks!