Most recent set the bison was released in was 1996. It may have been in add ons for a while but probably not for 20 years. It didn't show up in sets or add ons the last 3 or so Western series. It's heavy and therefore expensive to make. Not the rarest but pricey on the secondary markets.
I bought one within the last decade, I could have sworn it was from playmobil.us (as was), but maybe it was an ebay score. Funny I never realised they were relatively scarce.
Anyway I hope everyone watched that whole clip, she's got a great collection and fun attitude about the hobby.