I have noticed a lot of creepers and endermands spawning in bug ammounts compared to zombies, spiders, skeletons, ect. Horses spawn very frequently on plains as well. Also endermans on soulsand valleys. It happened to me on a realm

Steps to Reproduce:

1. set time to night
2. look around what mobs spawn and count them.
3. set time to day and locate a plains biome
4. take a look at all the passive mobs
5. enter the nether and locate a soulsand valley
6. take notes on the mobs spawning there

Observed Results:

Creepers and endermans are very common on night; more than any other mob.
Horses outnumber the pigs, cows, sheeps, ect.
endermans exist more than skeletons or ghasts on soulsand valleys

Expected Results:
The mobs should spawn and be distributed more uniform around the world.

Screenshots/Videos attached: See below. All the mobs are naturaly spawned and no spawning add-ons are used!

Notes: These obesvations have been done to a realm