Shoulda posted my comment about the boreal arboreal, the marten, here. Long sinuous body and tail, very fluffy. The red panda has a better (more teddy-bear-like) face, less beady eyes. But enough about how they look.
I just read about a six-legged pack predator from the movie Avatar: the viperwolf. Here's a model:
http://wesclei3d.cgsociety.org/portfolio/project-detail/904018/ I haven't seen Avatar, so wonder if they might provide a model for how the body moves. This looks like the paws are much less differentiated in function than a treecat's, so maybe not. But if the viperwolf demonstrated good running form, it might be something to start from? I'm obviously only looking at the kinematics part, not any aesthetics.
Hm, seems Avatar provides more than just the one as an example:
http://scilib.narod.ru/Other/Cinefex/Avatar.htm Early on, Cameron had conceived all of the terrain-based creatures as having six, rather than four, legs, before committing to the notion, however, he hired ILM animator Paul Kavanagh to create a series of walk and run cycles to determine if a six-legged creature could move gracefully and powerfully, or if the extra legs would inevitably create on awkward gait. The animation cycles proved the viability of the idea, and all of Pandoras land animals were endowed with six limbs.