Is Zen an extreme form of Mahayana? In this Partially Examined Life podcast one of the presenters stated that Zen is an extreme form of Mahayana. Is that true in any sense? Are there elements of Zen that could be classified as extreme? Or is the statement entirely wrong?
Extreme means "furthest from the center or a given point; outermost." Zen comes from Japan, which came from Chan in China which came from Mahayana in India. Thus Zen is far removed from the origin in India making it more extreme. I am a Zen practitioner in Florida of the United States. Here it is even more of an extreme since we are even further from the center point of origin. 

"the extreme south of Florida"

Or look at it this way, your arm is an extremity of your body. Zen is an extremity of Mahayana buddhism. One could say the arm and the body are one. And one could also say the arm is an extremity of the body. Both view points are correct.

Zen in Japan has a lot of added texts and stories. Check out the book "Zen Flesh, Zen Bones". The "Gateless Gate" is in that book. The Shōbōgenzō written by Dogen is a Japanese Zen text. These are texts native to Japanese Zen that you will not find in the Chinese/Indian/Tibet texts. As far as practice, meditation on a Koan is a good example of a specific Japanese Zen practice that you will not see in Indian mahayana buddhism