This rune effect summons a pack of zombie bears that charge forward for a short distance, stampeding over and through any enemies and dealing 235% poison damage to anything in the area. This is one of the Witch Doctor's more damaging attacks, and though it is short range, it hits across a quite wide space.
The bear effect was created by Julian Love and revealed at Blizzcon 2010, where it became something of a meme or running joke, where the Diablo III devs referred to it in interviews and panels with unbearable frequency. 
Slashdot: I was playing a Witch Doctor, and I used the spell that summons a zombie for you, and I got a rune that turned it into a swarm of bears. We got a real kick out of that.
Julian Love: Yeah! Zombie Bears! It’s kind of funny, that’s one of the skills that came up – the way we work is that we sit down and throw a lot of crazy ideas out, and sometimes it’s a little scary, throwing an idea that you think “I’m not sure how the group will react. This is kind of wacky.” And I remember sitting in the group and they were saying, “What are we going to do? We need something else.” And I said, “I’m not even sure why this works, but I’m just going to say it: Zombie Bears!” And everybody went, “Yes!” And that was it. Look at the way to it feels to play that skill in the game – it just sounds fun, right?
Two screenshots of the zombie bears. It's hard to get a good shot of them as they move quickly and have a skinless, bloody mess of an exterior. The bear-ness of them is much more visible in a video than a still image.