User blog comment:Martialmaniac/Inferred Nen Type/@comment-4932025-20160620002148/@comment-26871105-20160620010355

Actually, Youpi's shapeshifting is most likely a natural trait but on steroids due to his strengthening of it with his Enhancement (Enhancement improves the quality of something within and possessed by the user).

Meruem seems to be a Specialist with everything considered. First of all, he absorbs other people's Nen and their Hatsu when he devours them. Secondly, he is capable of utilising other people's Nen abilitlies (Youpi and Pouf's) on first try without any difficulty whatsoever. His photons are a combination of Transmutation, Emission and Manipulation (though mostly Transmutation) without any difficulty and prior training too. All of this seems to points towards the notion that Meruem is a Specialist. Because the only characters who were shown to be capable of not only taking other people's Nen abilities, but also instantly master them without any training too, are all confirmed Specialists (Chrollo and Hagya/Leol); and the only person who was shown to master their own Nen ability instantly right after devising it, was Pitou, who's a confirmed Specialist.