Rick and Michonne were hunting for weapons and Rosita decided that she would also take part — only she would do so on her own. While exploring the surrounding areas, looking for anything that would be remotely useful in the war against Negan, she encountered a walker.
However, it wasn't any old walker, it was one of the most horrifying walkers that we've ever seen on the show. In fact, it was so horrifying that it could give the well walker from Hershel's farm and Winslow the metallic warrior walker a run for their money. Props to the make-up department though — these zombies getting more grotesque every season.