Apple has released four animated sticker packs for use with the current iOS 10 beta. While these sorts of add-ons will be downloadable through the Messages App Store later this year, currently, you'll find them in the store on iPhone, iPad, and desktop. While you can see them if you're running iOS 9, the stickers can only be downloaded on the iOS 10 beta.
In addition to the Classic Mac pack, there's also Smileys, Hearts, and Hands. The latter three all made their initial debut with the Apple Watch last year, and allow you to send more animated expressions than standard emoji characters.
Later this year, developers and artists will be able to upload and sell their own sticker packs to the Messages App Store.
If you have an iPhone or iPad running the iOS 10 developer beta, you can download the stickers from the App Store now.