Okie dokie! things you will need:
A push pin
Jewelry findings: one small one and one bigger one (or a wire*)
Pliers : You can do it with one but its more helpful if you have two. They can be any size, I just have miniature ones.
First, I would open up one of the findings. I would say to do it to the thinner one since it is easier to open that way. Use the pliers to hold either side and twist them apart. It would look like this when you are done!:
Then, your going to need to use the push pin to poke a hole into the pony's ear. Don't do it too close to the edge because then it might just tear all the way through. Twist the push pin so that it is easier to push it through.
Take the finding that you have already opened and thread it through the hole. Now you can either close the finding, and open a new one, or you can put the other finding in then close it. Either way they should be connected like this:
After that you're done! just put it onto your keychain like a regular key!
*Note: If you are using wire then just thread them into loops and follow the directions like that! Also, you can find a bag of findings at places like Walmart, or really any craft store. They're usually like $2 for a pack like the one in the first pic. They come in different colors too (copper, gold) if you don't want silver!