One of the most notable characters to cross the stage of North America during the Pleistocene was the so-called American lion. For many, many years (although curiously, not when it was first described) the American lion was considered to be an actual true lion, Panthera leo atrox. A somewhat recently published paper called this into question.
See, all the extant pantherines (members of the genus Panthera and their closest relatives within the Pantherinae subfamily are very closely related and fairly recently separated from each other. It's very difficult to establish the relationship between just the four that are living today; fitting in extinct populations is even more difficult. It's generally believed that the tiger was the first to split off from the lineage that lead to the rest of them, then the jaguar, and the lion and leopard split from each other last. The morphological study that I'm referring to, by Per Christiansen and John Harris, suggest that the American lion is actually not a lion at all, and is most closely related to the jaguar--although not to suggest that it is a jaguar; just that it's more closely related to that animal than to other pantherines (as the modern lion is to the leopard, for instance.) This revives the notion that Panthera atrox is not a subspecies of Panthera leo but is in fact it's own animal, a separate type of cat unknown today, although similar morphologically to an extremely large lion or jaguar. Which of course are already similar to each other, just as all pantherine cats are very similar. From bones alone, it is very difficult to tell different types of pantherines from each other.
Of course, another study of mitochondrial DNA from both living and extinct populations of "lions" came to a different conclusion. Ross Barnett et al consider modern lions (of all populations), European cave lions, Berengian lions, and American lions to all be subspecies of Panthera leo. In their analysis, modern lions split from the other lineages at some time after becoming true lions, and did not mix since. This is a bit surprising, because their ranges would have bordered one another, if not in fact overlapped, but there it is. The European cave lion; the more recent version that is (Panthera leo spelaea as opposed to Panthera leo fossilis) shows remarkably little genetic diversity across it's entire range; from Portugal to the Yukon territory in Canada. In fact, it's believed that it probably underwent a genetic bottleneck, and a smaller subset of the population had to then re-establish the entire population. Of course, they actually leave off the question as to whether the American lion is a "lion" or something else; but curiously, if it is not, then neither is the European cave lion. According to their analysis, all three could be subspecies of Panthera leo or atrox could be a subspecies of a separate species Panthera spelaea or atrox could be it's own species, just most closely related to spelaea. Interesting indeed.
Panthera leo atrox, as they call it, on the other hand, shows signs of being genetically isolated from any other lion population for quite some time, despite bordering ranges with spelaea. They also come out as the closest to spelaea, indicating that atrox and spelaea came from the same population, which had no admixture with what eventually became modern lions, but separated and remained separated genetically for over 300,000 years. This is most surprising in that it completely does away with the idea that Panthera leo vereshchagini which is also called the Beringian cave lion, is a separate subspecies. But it also directly contradicts the idea that the American lion is not a true lion, and is in fact more closely related to the jaguar than to the modern lion, which in turn is more closely related to the leopard.
I've included an image that someone did of a Smilodon populator, the gigantic South American sabertooth who's just killed a peccary of some kind and is being confronted by an atrox; whether as a subspecies of lion or as its own pantherine species altogether, it would probably have looked very similar. The two animals are considered to be the largest cats that ever lived with P. atrox a bit longer and S. populator heavier and more robust.
For my purposes, I'm going with the atrox as a separate species angle, just because, hey, I like to think the North America is special and we had our own endemic critters, not just immigrants from Beringia. American lions in DARK•HERITAGE have very faint rosettes, not unlike that of young lions, over a tawny coat. They are not heavily maned, but do have a thickening of the hair around the neck. And like jaguars (and leopards) they are often subject to melanism; the condition that causes leopards and jaguars to instead become "black panthers." Atrox does not form prides, but is more social than many other large cats, which are solitary. Atroxes often congregate in smaller family groups; a parent and cubs, along with subadults, and occasionally one of the parents siblings often hunt together. These groupings are more fluid than prides, and occasionally mature individuals will go their own way when they tire of the relationship, or wish to seek out their own mates or other group. This is based on the same imperative that drove African lions to form prides; on open grasslands, larger groups are more successful against the large packs of dire wolves, packs of saber-tooths, or bullying short-faced bears, to say nothing of the difficulty in hunting such large animals as the various species of bison. But it's more accurately called a proto-pride rather than a true pride structure.
Atroxes in more forested regions tend to be more solitary, and have a social life not unlike that of the modern tiger.