So the Promethean Alchemist seems a bit poorly written. It grants the humunculus companion. It is worded and statted as an animal companion, but never actually points to the druid animal companion rules on how to interpret those stats (like the '4th level advancement' section). It just sort of assumes that you KNOW it works just like an animal companion, with the specifics listed as exceptions to that.
Are we to assume that it IS an animal companion, with the things listed as exceptions to that (like the ability to reanimate it and such)?
If that IS the case, could a Promethean Alchemist take Evolved Companion or Boon Companion? If it is NOT the case, is that just because the ability is not CALLED animal companion, so it doesn't meet the prereqs, or do you genuinely feel it shouldn't be considered one?
http://archivesofnethys.com/ArchetypeDisplay.aspx?FixedName=Alchemist%20Pro methean%20Alchemist
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And no it's not an animal companion
I don't think it should be treated as one because it isn't. It gains a bunch of things that AnCs don't get, and while similar, it is different enough to warrant being a new type of companion. Just the fact its natural armor doesn't go up each level and it has Sympathetic Alchemy is already different.
That and with the construct building and modifying rules (as well as the homunculus rules) allow you to do work to it, grant it spell-likes, armor and other things.
Yeah, it's a companion, but not an animal companion. But you should just ask your DM if you can take something like evolved companion or boon companion for it. It certainly wouldn't be overpowered.
While PFS legal, this is not a PFS friendly archetype and I'd probably stay away from it there. The main point of the archetype is to make constructs, which you can't do in PFS. You also can't use the rules for upgrading your constructs in PFS, which is about the only thing that makes the companion worthwhile.