As we near session zero day my group is asking questions about ancestries. My question to you is regarding rare Ancestries. Not info about the ancestries themselves, but rather the (rare) tag.
I found this info (PC2) about uncommon ancestries and heritages, and rare backgrounds.

But does something like this exist for a rare ancestry? I was hoping the Ancestry Guide would have something.
Generally uncommon and rare things just need a good story justification worked out between the player and GM. Some rare feats and items and things are really only useful in the specific adventure they were made for, but i think of uncommon/rare as just guidelines for GMs to determine how appropriate things are for the setting.
I hope I understood your question correctly.