The gross majority of Twitter users are the personas of humans and brands, of course, but there's a small group of Twitter accounts that aren't, and they are the best ones. They are the bots, and they are accounts controlled by software to do various things. Here's three of my favourtes.
Acrotowne is a simple game built by @zhaytee. First, it posts a random string of letters, such as "SATW". Then it's followers may send it a direct message with what they think those letters could stand for if they were an acronym. Finally, after a set amount of time, all the suggestions are anonymously published, and everyone who submitted a suggestion may vote. The one twist is that if you don't vote, any votes for your suggestion are nullified.
ANAGRAMATRON, by @cmyr, always tweets in pairs, and all it's tweets are retweets. It searches all recent public tweets for multiple tweets made up of the same letters, and then retweets them both. What you end up seeing in your stream are two totally unrelated tweets that just happen to be anagrams of each other.
Moon Shot Bot
Lastly, my all-time favourite: Moon Shot Bot by @tinysubversions. In 2015, the Project Apollo Archive uploaded 14,222 photos of the Apollo moon missions to Flickr. This bot just picks a random image and tweets it, posting four per day. Because of the formats used back then, they're all squares, and they make a nice addition to your stream of content.