I at first thought this was a biomes o' plenty problem, but even generating a default minecraft world, normal passive mobs (pigs, chickens, sheep, cows) spawn extremely rarely or none at all. I have yet to discover even flying around multiple generated worlds a single cow. When I did discover creatures they were in villages. I also thought maybe the spawn rate was simply too high for the unicorn mobs, so I edited the config file (can't use the config ability in minecraft as it is greyed out) to less than half, close to 1/3rd the spawn chance of each mob and there was no change.
I am running the latest Minecraft, 1.16.4