That's right, there is no monster in TotS with the Serpent's stats. The Archivist's stats are just a different combination of Zeno's. (Zeno initially has 8000/5000/1000; after debuffing he has 800/500/100, as the Rock Giant. The Archivist has 1000/800/500.)
Two other DROD RPG monsters have unique stats not taken from TotS:
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×1. The third encounter with the Pirate Captain (170/240/110). (In the first two encounters, he has the stats of the Golden Knight in TotS, 120/150/50.)
2. The Wubba (10/0/320)
And TotS has one other monster whose stats were not used in DROD RPG. (Yeah, it would have been fitting if they'd given these stats to the Serpent....)
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×The bonus fight with Zeno at the end of the "back side" of the tower. His stats are 44000/27500/5500, but everything in the back side is multiplied by 44, so his base stats are 1000/675/125.
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50th Skywatcher