Much like leather is a co-product of the cattle industry, castoreum was once the natural co-product of the beaver trade (via the Economic History Association). From 1670 to 1870, beaver was an economy unto itself in both Europe and North America. Fashion trends involved all kinds of beaver pelt items, including ladies' coats and gentlemen's hats. Once the beavers had been slaughtered for their fur, the remaining castoreum sacs could also be harvested and roasted over a wood fire to further augment their unique aroma and flavor (via Mental Floss). 

The beaver trade long gone, anyone needing castoreum today must procure it from living beavers by "milking" them for this sticky substance. The process involves anesthetizing the beaver and having a person massage the molasses-like goo from the sacs. Currently, only around 300 pounds a year are consumed. While that sounds like a large amount, when compared to natural vanillin at more than 2.5 million pounds per year, it's a drop in the (castoreum milking) bucket.

Since veganism defines practices like these to be both animal cruelty and exploitation, even these more ethical castoreum extraction processes don't meet vegan standards.