: TL;DR they are the result of descent with modification.

: Jackals didn't descend from any earth species. They don't have a common
: ancestor with anything on earth. Unless this common ancestor was some
: spore, something pre-bacteria/archea/viral. If this was the case they
: still wouldn't be members of any of those categories (clades) because
: those clades are derived from the, supposed, common ancestor-spore-thingy.

: A simple tree (that assumes common ancestry) would be: spore-thingy
: |
: |- lots of things that we don't know about -> jackals
: |
: |- eukaryotes -> metazoa -> amniotes -> sauropsids

: jackals are not sauropsids

Exactly. As an extraterrestrial species whose descended from earlier life forms on Eayn, Jackals aren't part of any clade of Earth organisms even if they have some superficial similarities to Earth avians (John said they reminded him of the "carrion birds" on Reach, which, unless they were imported from Earth, technically aren't part of the taxonomic class Aves either). The reason they have avian-like features is because of convergent evolution. Many millions of years of change and adaptation created a taxonomic group of animals on Eayn that just happened to share many of the same features of Earth birds (actually, they're more like the more primitive, now-extinct birds of the taxon Enantiornithes). Technically, Jackals aren't even technically "animals" as the kingdom Animalia is an Earth taxon; evolution on Eayn (and Doisac, Sanghelios, etc.) produced a kingdom of heterotrophic organisms that share all the same basic features of animals, so are for all intents and purposes "animals." Perhaps evolution on other worlds in real life might, perhaps due to biochemical necessity, operate by the same principles as well as produce life that has the same basic cellular structure (prokaryotes and eukaryotes), fits into the "heterotroph" and "autotroph" categories, and has kingdoms with roughly the same general features as Earth kingdoms, the their equivalent of Animalia having the potential to produce chordates, specifically vertebrates. In other words, it would be convergent evolution writ large, applying not just to two species but to the highest taxa.

Now, on to Jackals and this "subspecies" or whatever the Halo 4 Jackals are supposed to be. Here's a comparison between the Jackals as portrayed in Reach and how they're portrayed in H4:

While everything below the neck is practically identical, their necks are much larger and, more importantly, the skull structures are radically different. H4 Jackals have a noticeably different cranium with no protruding snout-like maxilla, their mandibles are far larger and protrude in a severe underbite, and their dentition is much different, with much larger teeth than the "standard" Jackal. Now, evolution works slowly in nature, and such radical differences in physiology within a lower taxon, even if only limited to the skull, are essentially unheard of among vertebrates. The H4 Jackals are supposed to be the same species, just a subspecies like the Skirmishers. However, the only incidence we know of where a subspecies has noticeably different body structure from another subspecies is the domestic dog. Years of selective breeding turned the grey wolf into the myriad breeds of dogs we have today. The wolf skull went from this:

To these (a pug and a Great Dane), among others:

But even then, the difference is in the overall proportions (the Great Dane's skull is very wolf-like, while the pug's has a reduced snout/flatter face, larger eye sockets, and is much smaller overall). While it does show how much variation can be produced within a species, it's still totally artificial. In nature, there is very little morphological difference between subspecies of a given species. The natural subspecies of Canis lupus are all largely indistinguishable from each other. Same for the various tiger subspecies (e.g., Bengal, Sumatran, Siberian). The untrained eye would be unable to tell them apart.

At the level of genus, there isn't that much morphological difference between member species. Homo erectus and Neanderthal man probably didn't look too radically different from modern humans. Most people probably couldn't tell the difference between the members of the genus Canis (e.g., gray wolf, coyote, jackal) or between the members of the genus Felis (e.g., domestic cat, sand cat, wildcat). Skeletal structures between member species of a given genus are very similar, often to the part where you can't tell them apart unless you're trained to do so. Can you tell which of the following is a lion (Panthera leo) skull and which is a tiger (Panthera tigris) skull, or do they look nearly identical to you?

You have to go up to the taxonomic level of family to get differences in morphology and skeletal structure between member species of the same taxon. The family Canidae contains the genera Canis, Vulpes (true foxes), Cuon (the dhole), Lycalopex, and several others. The family Felidae contains the genera Felis, Panthera, Leopardus, Lynx, Puma, and various others. The family Hominidae contains humans, chimps, gorillas, orangutans, and extinct genera like Australopithecus. But even then you can tell they're still closely related to each other. Dog. Cat. Ape.

The differences between the common Jackal and the Ibie'sh Jackal are so stark that it's clear that, even though they share a common ancestry, they are at best members of the same family, and possibly are only of the same order. They are clearly not of the same genus, much less of the same species. Only radical and advanced genetic engineering could have turned the common Jackal into the iguana-with-an-underbite we see in Halo 4, and even that would be a stretch. I could maybe buy the Skirmisher being a closely-related species (being to the common Kig-yar what the Neanderthal was to us) as they retained the same basic head structure and avian appearance, but, unless they're some bizarre artificial creation, I can't buy the Ibie'sh Jackal being of the same species. If they're a natural variation, they're a different species whose most recent common ancestor was at least a few million years ago, if not more. Same for the new Grunts; while they retain the same basic body shape, their integument (leathery turtle-/crocodile-like skin vs. the crab-like chitinous exoskeleton of previous Grunt designs) are too different for them to be even of the same class, much less the same species, plus the digits on their feet are all wrong.

We all know what's really going on here. Somebody at 343I felt like he shouldn't be constrained by Bungie's artistic legacy and wanted to flex his creative muscles, and was given carte blanche to redesign things as he saw fit, so long as the end result kind-of sort-of resembled a Jackal. It's artistic license, plain and simple, and now that these freakish abominations are part of the canon, they have to find a way to shoehorn them into the fiction. Artistic license is the only way you can explain this progression in Jackal design since Halo 1:

Yes, Bungie did redesign the aliens a fair amount, but at least most of them looked roughly the same from game to game, and the differences in design could usually be chalked up with advances in graphical technology (better textures, higher polygon counts, etc.) allowing them to offer more detail or better flesh out their aesthetic vision. Except for the Brutes, none of the visual changes made the aliens suddenly look like members of a different species. We certainly never got a wholesale restructuring from the ground up of all the major assets, at least not to the degree we saw in Halo 4. 343I left no stone unturned. If there was something that could be redesigned, they redesigned it, even if the end result made no sense, looked radically different from what came before, and did not mesh with either the fiction, Bungie's artistic legacy, or even common sense. There were a few weapons that got away mostly unscathed, but most things were redesigned, many radically so, well, just because. I hear the person responsible for all those redesigns in Halo 4 is gone now, so hopefully in Halo 5 they'll bring back the classic Jackals and Grunts.