There's two ways to approach this - if you just need this for a single encounter, and just good enough to use for initiative and other basic things, then use "Basic Monster" - that'll let you just fill in values for the AC, HP, saves, etc. and go from there.
Detailed Monster will give you a more functional end product. Sounds like this is a custom monster - so for the Ancestry, choose Custom NPC - the very first item in the list, and then start filling out stats, and adding various abilities. You'll likely end up adding several custom abilities, so that you can fill out the unique things this monster can do.
In PF2, monsters only get major abilities from classes, if they've got some class-like things. Often, what they get is a variation of the PC version, re-designed to be just what's needed for one monster for one encounter - nothing long-term, and with some of the PC complexities removed sometimes.
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