None of these mobs spawn in my world. Even after doubling spawn rates I have not seen a single one of these. Are they extremely rare? Because every other mob spawns fine, even iguana though its a little rare. In the nether I see nightmare spawn many times but never reaper.

I checked all these in their specific biomes.

yeah they work now this was very old

No they are not that rare except for the rhino. Reopen if that is still the case in vanilla biomes on the latest version.