NPCs emulating character classes are many. A very incomplete list would include: Abjurer, Archer, Blackguard, Bard, Champion, Conjurer, Diviner, Druid…
You get the idea.
Among the monsters there's the Hobgoblin Iron Shadow and the Shadar-kai Shadow Dancer, but I'm struggling to find Monks among the NPCs in the sources I own so far. I'm hoping to represent various Monk subclasses in NPCs, and I'm looking for the balance that I attribute to the officially-made NPCs.
Best Answer
This search at D&D Beyond has several, including stat blocks from the Monster Manual, Candlekeep, Princes of the Apocalypse, Call of the Netherdeep, and Lost Laboratory of Kwalish.
Searching instead for “unarmored defense” yields many more results, including stat blocks from Planescape, Volo’s Guide, Dragonheist, Eberron, and Storm King’s Thunder.
Worth highlighting here is the adventure “The Book of Inner Alchemy” from Candlekeep Mysteries which contains several stat blocks for different levels of NPC monks.