LibRareIds-1.0 returns a table of all rare/rare-elite NPCs in the game. It's a workaround for situations where you can't use UnitClassification (e.g. COMBAT_LOG_EVENT). LibRareIds-1.0 was inspired by LibBossIds-1.0 which is basically the same for dungeon and raid bosses.
Let's find out if the mob with the id 10558 is a rare:
print(LibStub("LibRareIds-1.0").Data[10558] ~= nil) -- true
And this is how to retrieve the id from a GUID:
local GUID = UnitGUID(unitID)
local mobID = tonumber(GUID:sub(-16, -12)) -- WoD 6.x
If you enjoy this library please support it's development. Thanks!