You're positing a split a very long way back. Things like the quadrupedal form and the pentadactyl limb developed so far back you'd almost have to posit another Class entirely to make it reasonable.
Convergent evolution is all very well as an explanation, and happens reasonably frequently in nature, but what you're saying there is that these creatures are known not to be mammals. What are they, then? Saying 'what might this thing have evolved from' is far too broad without more of a description than 'they have six legs and live in small groups'. I'm assuming from the name that they're about cow size and morphology, but is that correct? Are they six legged horses, dog-size cockroaches, snakes with a compensation problem or elephants with a limb surplus?
Assuming these creatures are vertebrates, and therefore chordates, you have an interesting problem regarding the attachment of the third set of limbs. Do you have a second set of scapulae? Something approximating a second pelvis? A swung-cage like structure suspended from reinforced vertebrae developed from the last two or three ribs of something otherwise roughly mammalian?
If they're not chordates as we know them, you have further options. Insects already have six legs, have done so for a very long time, and their main reason for being small is a lack of lungs - the oxygen in our atmosphere isn't sufficiently concentrated to allow them to function. Positing that a large species of insect developed internal lungs of some kind, perhaps by merging bundles of their tracheal tubes and enlarging a couple of their spiracules (giving you an insect with proto-lungs and 'nostrils' on its 'shoulders') would mean that as the oxygen levels dropped, they were able to stay large and perhaps even grow, eating vegetation which the other species of insect were no longer able to consume as effectively. Group size would drop due to availability of food and costliness of raising young, the transition from oviparous to ovoviviparous (and perhaps to viviparous given time) has happened a few times in several different families, and various creatures give milk - the Pacific Beetle Cockroach, for one.
If you don't fancy insects but the extra scapula thing bothers you, how about a parallel evolution of the endoskeleton? Something that doesn't have a spine, pelvis, ribs and scapulae, but instead depends on a mesh of hollow long bones pivoting on rounded cubes (think a body composed entirely of carpals and phalanges); their organs protected inside a cage of bone while their nerves are distributed rather than having a CNS to speak of. They might be able to carry quite a bit of weight, as they would have to be tied together internally with tendons and would therefore be motile Hoberman Spheres. The legs would therefore just be the same construction with a set of much-enlarged cubic pivots within the body, two greatly-lengthened internal bones and a 'club foot' consisting of a very large pivot bone with padding and keratinised skin. That could have evolved from... well, almost literally anything you like, because the vertebral column developed so long ago.
My point isn't really to seriously suggest the foregoing, but to point out we could really do with some more information.
On a more serious note, this might not be as good an idea as you think. Hexapedal creatures are cool, but six legs is actually not as efficient in large animals as four legs. Legs are heavy and require lots of nutrients to maintain, while providing only small stability and motility benefits. They're also slightly harder to coordinate.
See this for how hexapedal creatures have to be constructed to move efficiently.
