Describe the bug
I have a few mods that adds own mobs to the game. But it's insanely hard to find them except mobs from your mod.
My mods that add new mobs:

• Animania
• Ice and Fire
• Grimoire of Gaia
• Fish's Undead Rising
• Mowzie's mobs
• Project: Vibrant Journeys

Also I have a few mods that make changes to existing mobs:

• Rough Mobs Revamped
• Infernal Mobs

And a few others, that may do something:

• Hardcore Darkness
• BattleTovers
• Better Caves

To Reproduce
Steps to reproduce the behavior:

1. Create a world
2. Just fly around, generally during the night
3. Notice only your mobs, and a few from animania. Very rarely from other mods ( I found only one mob from Grimoire of Gaia during I was flying for 30 minutes ) or even vanilla mobs (1 zombie and 2 creepers)

Expected behavior
I saw the similar issue reported here, but there u said that none else reported such thing, so I decided to do it.
I'm gonna try to set "allowDespawning" to "true" for all mobs and look what will happen. If I helps, I will write about it here. (It may take a while cuz I need to reset all spawning rates from other mods - I decreased them as much as possible.)

Versions:

• Minecraft Version: 1.12.2
• Forge Version:
• Mod Version: 9.0.1

Additional context
Firstly, I tried to find mobs from Grimoire of Gaia. When I couldn't do it, I contacted developers of this mod. They said that their mobs may not spawn cuz of the maximum number of mobs can spawn in one chunk and this number may be reached with other my mods that adds mobs. So I decreased spawn rates of all mods, but it didn't help.

I've tried flying in new world with allowDespawning turned to true
And it didn't help at all
Still your animals and animals from animania
3 mobs from ice and fire
2 creepers and 2 sceletons
some archers and a few pharaons from Grimoire of Gaia
and that all i found despite i've been flying for an hour.

I noticed that situation in desert and cold biomes is not so bad. But "green" biomes are almost emply and dead.

I'm gonna try to disable your mod and then try to fly in new world.
Honestly, I hope the problem is in your mod cuz if it doesn't then i dunno what to do.

I've spent a lot of time and it looks like I got closer to what causes this issue.
And it seems like your mod doesn't do it.
I disabled almost all my new-mobs mods.
And stall I couldn't find any of vanilla hostile mobs anywhere.
So I tried to think what else can cause this.
And looked at all my 'performance' mods.
foamfix

Speeds up entity lookup by optimizing ClassInheritanceMultiMap.getByClass. [default: true]
B:fasterEntityLookup=false

Fixes the server not removing unloaded entities/tile entities if no chunkloaders are active. Thanks to CreativeMD for finding this! [default: true]
B:fixWorldEntityCleanup=false

As u see, I've turned this options off, and I finally found some hostile mobs!
I dunno which of this 2 options cause the issue, but I hope everything will be fine now.