Invisible Mob Nether

I encountered an invisible mob in the Nether.

What I expected to happen was...:
-

What actually happened was...:
I was walking in the Nether and suddenly got attacked by an invisible mob. I've looked it up and a minecraft mod said that somebody would've 'spawned it in' but I'm playing singleplayer, nobody else has access to my PC and I didn't load that chunk before.

Steps to Reproduce:
1. Go into the Nether
2. Go into a Bastion Remnant
3. Get killed by the invisible mob 🙂