I think if you showed the card Beorn the Fierce to a time-travelling Magic: The Gathering player from 2003, their poor little head would pop. Five mana for a trampling 6/6 that also gives your bears a huge buff, turns other creatures into bears, and draws you cards? Nowadays, that's just Tuesday, but the arrival of Beorn from the Hobbit set has been sufficient to set several bear-related Magic cards a'spiking.
For instance, Words of Wilding has doubled in price from $3 to $6. This card is particularly synergistic with Beorn, since you can use the card draw from Beorn to make more bears, or skip out on your draw step to ensure you hit the Beorn trigger.
The largest price spike we've spotted is for the Legions card Caller of the Claw, which has jumped from $7.10 to $29. Caller of the Claw is an interesting bit of board wipe protection, as you can flash it in and make one 2/2 bear token for each creature you just lost.
Beorn makes this card at least twice as good, as you'll now have 4/4s instead of 2/2 bears. Crucially, as well, it keeps you at a critical mass of bears, so that you won't miss out on that valuable extra two cards.
Another old card which keeps you swimming in bears is Bearscape. This card was $4.50 before the Hobbit released and has spiked to $15 now, according to MTGGoldfish. Bearscape is an enchantment with an incredibly simple effect. For the mana cost of the classic card Grizzly Bears, you can exile two cards from your graveyard to make yourself a 2/2 bear.
Usually you'd do this if you have Beorn and just one other bear on the field to ensure you get the trigger, but it could also just be a nice easy way to build up a whole board of blockers.
Bears are a surprisingly popular tribe in Magic, and according to EDHREC Beorn the Fierce is actually the third most popular commander in the set (just behind Smaug and Legolas' dad). Of course, it really doesn't hurt that Wizards of the Coast keeps printing extremely strong bear cards.
We already have Ayula, Queen Among Bears as the premier mono-green bear commander but, even though he's significantly more costly on the mana front, Beorn may actually be a stronger, more consistent option. It's hard to ignore the benefit of that extra card draw, even if Ayula's constant brawling is ultimately going to be more fun.