How about this?

In a human universe, scientists are figuring out how to travel to different universes. After many attempts, the scientist just gave up. As they left for the night, and the janitor was closing up, the machine turned on. A portal to Ponyville was opening. A few seconds later, Fluttershy's head pops out. Looking curiously around, she spots a cat laying asleep in the corner.
:Awww you poor thing
The cat wakes up. Scared at the site of Fluttershy, the cat jumps up and gets stuck on the ceiling. Fluttershy smiles as she flies to grabbed the cat down.
:Looks like you just saw a ghost little guy
Fluttershy goes back through the portal with the cat clawing on the ground begging not to go. The portal closed. The next morning, the scientists walk in, one of them spotted the claw marks on the ground
Scientist: Uhh were's Fluffy?
Meanwhile in Ponyville:
Fluttershy gives the cat some food
:here you go little fella


THAT'S ALL I HAVE SO FAR

Scientist open the security camera files and see what happend
They redo the steps that the janitor did and the portal opens again.
Only problem is... they do not have a way to enter it, they are only able to walk through the portal but not enter Ponyville. They try with a labrat wich has no problem to enter ponyville. After experimenting with other animals they figure out that they have created a portal in which only animals can enter. So they invent a machine that turns them into ponies.

As they step into Ponyville, they're amazed at what they see
All of a sudden Pinkiepie pops out of no where
Hi, I'm Pinkiepie, I haven't seen you ponies in Ponyville before, you must be new
Scientist: Uhh, I'm Greg, and these two are my assisstents, John, and Kelly
John and Kelly: Hey *waves hooves*
Greg: Have you seen a black cat, about this tall? *holds hoof a foot above ground*
Cat? Nope, but I think my very best friend in the whole-wide world may have seen it.
Greg:
John: We don't have much time Greg, the portal will close in 2 hours.
Greg: