This page is for listing the various theories on the content and origin of the Voynich Manuscript: by its nature it is unlikely to be complete.
The following list is based in part on the 'Authorship hypotheses' section of the Wikipedia page on the VM 
- Wilfrid Voynich and William Romaine Newbold believed the manuscript was written by Roger Bacon.
- In 1987 Leo Levitov published 'Solution of the Voynich Manuscript: A liturgical Manual for the Endura Rite of the Cathari Heresy, the Cult of Isis.'
- 2014 A Preliminary Analysis of the Botany, Zoology, and Mineralogy of the Voynich Manuscript - article by Arthur O. Tucker, PhD, and Rexford H. Talbert.
- Leonell C. Strong considered the VM involved a "peculiar double system of arithmetical progressions of a multiple alphabet".
- Raphael Mnishovsky was a noted cryptographer and it has been argued he created the document.
- Voynich himself fabricated the manuscript - see Case against Wilfrid Voynich authorship
With most of the above the dating is problematic - given that the manuscript has been dated to the early 15th century.
- Hoax (of various kinds)
- Constructed language
- Micrography - William Newbold (since disproved, and improbable given the length of the text).
- Obscure natural language
- Hybrid language
- A 'presentation document' by an astrologer or similar.
- Someone not literate in the script miscopying an existing text.
A number of the above possibilities are in practice, excluded on the grounds of the dating of the manuscript (even if some were based on incomplete information at the time - for example Voynich and Newbold did not have access to modern dating methods). For most of the others, given the absence of context and related material the verdict is the Scottish 'Not proven.'
However - the manuscript does look like 'an ordinary medieval manuscript' of the time it was created.
See also Voynich Manuscript timeline