Strange plants, unrecognisable animals, and an unknown language: it is not surprising that some joke that the Voynich Manuscript was written by aliens. One of the longest-running joke theories is that the Manuscript is the illustrated diary of a teenage space alien who left it behind when he returned home from Earth.
Is this theory realistic? Probably not. But still, aliens are the first thing most people think when they see the Manuscript for the first time. And it is fun to think about.