Anti-meta is a pretty broad term, as dynamic as metagames are since how a metagame evolves determine how something is or becomes anti-meta. An uncommon set wont guarantee that something is anti-meta right off the bat, it got be a set that can take advantage of common checks and counters or teambuilding trends. Banded ttar is a pretty neat example of it since common defensive mons that think they can switch just like that just to get blasted.
Other example of anti-meta sets would be: sub salac SD lando-t, banded kyurem-b, calm mind + 3 attacks celebi, calm mind encore mega alakazam (or other moves), crawdaunt in general (with mega altaria usage dropping) and some more stuff.
An example in previous metas of how something was anti-meta is weavile. During XY it was really uncommon but it still was a living pain for offensive teams and something people just didnt prepare for. Outspeeding greninja means the fastest offense staple just couldnt really handle it and it goes like that. Then ORAS made weavile much more common, people preparing for it and then stopping being anti-meta.
Lengthy response but isnt a simple question :V