There are only three illustration of animals in the herbal and pharmaceutical section: a pair of snakes, a dragon and a frog.
The snakes and dragon are very common in medieval illustrated herbals, but the frog isn't.
In fact, beside snakes and dragons, one would commonly see scorpions and dogs, and in almost all cases the animal has something to do with the medicinal properties of the herb.
What would the frog mean?
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This is clearly the same herb as on f32v:
The "artist" has taken the trouble to bring in one leaf and one flower to show that it's the same.
Which herb would have something to do with frogs?
One option is given by the Tractatus de Herbis tradition. There is usually a frog ('rana') drawn closely to a herb called 'ranaria'. This is because the books show herbs and animals mixed, and they are organised alphabetically. Here's an example from Munich CLM 28531, which I have rotated for layout purposes:
The herb does not look at all like the herb in the Voynich MS.
Another incidental example I found is in a Latin illustrated copy of Dioscurides, also in Munich: CLM 337:
Now this one is a bit more interesting as the herb does bear a passing resemblance to the VMS herb.
This one is Cyminon Agrestis, which in the more common Greek versions is
Κύμινον άγριον
(hope that comes out).
After this, it gets more difficult. In Greek copies of Dioscurides (I only checked the online ones in Naples and in New York), the herb looks very different, and there is no frog. The reason of the frog seems to be a standard venom-related issue, but I haven't seen any specific reason why a frog and not a dragon/snake/scorpion.
Unfortunately, the scan of CLM 337 is not of the best quality, and the Beneventan script takes quite a while to get used to.
So, no great answers yet, but perhaps someone else knows a bit more.