Saw someone post this on reddit. Pretty sound idea and it's been mentioned a couple times before, but I guess I'd never connected all the dots.
Basically, the idea is that the ability of Tserr.'s horse Nen Beast is to turn liars/those who betray the prince/etc. into Nen Beasts for him.
The horse Nen Beast says to Theta:
And Tserr. does seem to enjoy transforming/modifying the human body:
And something I'd forgotten about was the bloody mess in his suite:
We still don't know who that was, but immediately after that, he created the "Specialist Nen Beast" (as Theta calls it) that gave him the time/foresight ability.
So yeah. I'm not sure whether the Specialist Nen Beast is related to that dead person or not, but the idea of Tserr. getting a multitude of Nen Beasts is definitely... interesting lol.
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However, when he manifested the new one, it looked eerily similar to when Pitou came up with her ability (though Theta says that Tserr. conjured his "without intent"). Top is Tserr.'s, bottom Pitou's:
So it's probably unrelated to the dead person and the whole creating more Nen Beasts thing, but something to consider.