New World

To avoid as much confusion as possible, I'll be explaining a few things here.

When Steve got to Equestria, part of his world came along. After a while, that part fused with Equestria, changing the physics a little bit and making it able for monsters to spawn. But because of the huge amount of magic in the air, the allready large amounts of monster and that this is Equestria and only a small part of Steve's world came along, the monsters spawn a lot less frequent and in a larger range. Their behavior and other things have also changed a bit:

Zombies and skeletons will only die when the brain (skull for skeletons) is destroyed. Zombies will also try breaking windows. There are also zombie and skeleton ponies.

Spiders can think, but only barely. They can sense when the enemy is too too strong and are able to feel fear. Spiders don't despawn and gather in forests and caves during day. Oh yeah, they poison their prey before killing it.

Endermen are far more intelligent and dangerous, but will only fight when neccesary.

Creepers are a lot more scary now. And when I say a lot, I mean it. They are a lot more violent and can trick you. They also attack other monsters.

Spiderjockeys, are very intelligent and can think normally. It's basicly a intelligent skeleton who has tamed a spider.

Silverfish look like, well... Silverfish. They can eat through almost any material and always travel in groups.

Slimes have a lot more variety and don't jump.

Bats don't spawn because they are natural in Equestria.

Witches are a lot more evil and look... different. Yuck.

Now to the normal creatures:

Cows, sheep, pigs, chickens and wolves don't spawn.

Villages are very rare and the villagers look more like normal humans (no, they don't look like Octo anymore.) and can talk.

Iron Golems guard the villages but some of them roam Equestria, attacking all momsters they come across. Their faces look like normal faces.

And now to the physics:

They haven't changed (yet).

I hope this will help avoid confusion as much as possible.