Hi! There have been a few threads on marginalia. No specific discussion on "why rabbits", so more info is welcome but meanwhile, you can get started on some more general background here:

What's up with marginalia in general?

• How were bored medieval monks able to get away writing complaints and making off topic illustrations (some NSFW) in the margins of their transcriptions?

• Can anyone explain why things like this show up in medieval manuscripts and even copies of the Bible?

• Is there a context to this medieval picture?

• How Would a Medieval or Ancient Scribe Go About Writing A (Secular) New Work?

• thread in High and Late Medieval Europe 1000-1450

Specific themes

• Women/mermaids/sirens How come in Renaissance style paintings (and even further back with Greco-Roman art) all the women who are nude are hairless? Was it an attempt to just show purity, or did higher class women shave their bodies?

• Snails Why are there so many medieval paintings of people battling large snails?

• Monkeys What did people think of the anatomical and cognitive similarities between humans and apes before before evolution?

Tangentially-related threads

• If vellum was so expensive in the Middle Ages, why do medieval books have such large blank margins? - more about the margins themselves: why were they so large and various uses for them (including illustrations)

• Why was medieval illustration (especially in calligraphy) so shitty by standards of other periods? Was it only because it was made by (essentially) clerks who did it as an afterthought of calligraphy, or was there more to it? - not so much about the content but more the artistic quality of marginalia