The chances that a monk translating greek fables would have been out in the wilderness to study large rodents are slim.

They could know beavers from the kitchen, though. In medieval Europe, beaver was the only mammal that could be eaten while fasting, as it was seen as a type of fish due to the scaly tail and semi-aquatic life. Fish is ok to eat while fasting, so beaver tails were a mainstay in many monasteries. And we're talking about a kitchen where you don't take the pre-cut beaver steaks out of the freezer, we're talking "here's five whole dead beavers that just came in, we need to start chopping these bad boys up right after the morning vigil." They knew fully well what they looked like, they just had to look at their plates to study the tail in all its intricacies. Yet they drew these very tails as furry, branched scrollwork things. Because realism just wasn't something people bothered with at all in medieval art. It was entirely formalist. You can tell it's a beaver not because it looks like a beaver, but because it bites its balls off, and it will always only be shown looking like a weird dog-weasel biting its balls off because that's the proper way to draw a fucking beaver. In artwork like that, you can't tell one dude apart from the other by looking at their faces because everybody has the exact fucking same face, but you can always say exactly which aristocratic rank one holds and maybe even which house they're from because they're shown with the appropriate crown and coat of arms. Same for religious figures - you can always tell who's a saint and how saintly they are by looking at their halo, and each saint is depicted with some icon unique to them, an item they're holding or something like that, but apart from these signifiers they look like clones.

Doing art the classical way, trying to make it look realistic, studying anatomy and understanding how the muscles run under the skin and all that other work that goes into making realistic art, that wasn't a thing in Europe until the 13th century came around (or the 15th if we're talking outside of Italy). That was just lost to time. What mattered in art was convention, not expression or depiction.