Twitter is known for testing out features with a small group of users before rolling it out to their entire user base. It appears that they are doing that yet again with a new dedicated GIF button for their Android app. A few users are seeing a GIF button in the compose window of the Twitter for Android app, next to the media and poll options within the app. It appears that right now, the GIF button will allow you to pick a GIF that's in the "trending" tab, or pick one that's in a category, which are set based on the mood you're in.
Currently, GIF's aren't hard to find and share on Twitter and elsewhere. Many people use Giphy, which have many GIF's available for download, making it easy to send in a message or share on social media. Twitter recently added the ability to auto-play GIFs in the timeline, in addition to videos. So seeing a dedicated GIF button appear in the compose window isn't a huge surprise, when you think about it. GIF's are fun and entertaining, and becoming a big part of Twitter as of late.
The button quickly disappeared for the users that did have it yesterday. So it appears to be on a server switch, where Twitter can quickly remove it without pushing out an update. We aren't seeing it here on our install of Twitter for Android ourselves, but it's likely something that might be rolling out in the near future. Twitter has done this before with different features. There was a timeline full of users that you had favorited. So you could see all of your favorited users in one timeline. Twitter quickly got rid of that - likely after some user feedback - and we haven't seen it since. So while this is still in the testing phase, it's possible that we see it rollout to the app in the near future, but we could also see it disappear forever. We'll have to keep an eye on this and see what happens, it would be a great feature to have baked into the Twitter app, however.