Fuller House's second season just finished filming, and we've already got some exciting hints about the show. New Kids on the Block will make a cameo, and Jodie Sweetin has said they'll have a number of exciting guest stars.But she didn't mention this one.John Stamos posted a photo of himself with a monkey on his head to Instagram, Hello Giggles reports, along with a cliffhanger caption.“Is that Jesse and Becky’s new kid? Watch #fullerhouseseason2 to find out!” he wrote.We're going to venture that the answer is "no," but this does imply that a non-human primate will somehow find its way onto the show.Some Instagram commenters are speculating that the monkey is Katie, the same one who starred as Marcel in Friends, People points out. (Katie has also appeared in a photoshoot with Kendall Jenner.)When Sweetin said we could expect celebrity cameos on Fuller House's second season, we pictured people. But we suppose Katie counts as a celebrity.