Illuminated manuscripts have lots of different creatures depicted on the margins. Some of them are real, others are not. One would be ready to find a knight fighting a dragon on the margins of a Medieval book. Knight fighting a snail? That’s some else!
It appears that defenseless snails had more of an allegoric meaning. Lilian Randall, a book historian, sifting through manuscripts in the 1960s realised she should research the matter. She developed a theory that snails may have been a symbol of oppression and an insult.
Find out more in this video by Vox:
There is a nice blog dedicated to manuscript art: http://jessehurlbut.net. For some examples of snail depicted in the manuscripts (however, not fighting any knights), use this link: http://jessehurlbut.net/wp/mssart/?tag=snail