Aurox Armoured Transport
The Aurox is a transport vehicle used during the Great Crusade and the Horus Heresy by the Solar Auxilia and the
Imperial Guard IMPERIAL ARMY. Both it and its tank variant (see below) are blatant nods to the bygone days of 40k's past when, prior to getting their own vehicle lines, the Imperial Army just used Rhinos and Predators, that probably makes the Dracosan their equivalent of a Land Raider... Actually, the Imperial Army had tons of Land Raiders. It was even better in Rogue Trader, though, as the Army and later Guard spammed Predators and used squadrons of Land Raider Proteus instead of Leman Russ tanks. In any case, the army changing from cheap Rhinos and Predators to clearly more complex and therefore probably more expensive equivalents is too dumb to stomach BUT they look cool so let us just enjoy.
Basically a Rhino stripped of all the advanced tech and fancy bits, the Aurox served the Imperial Army as the Chimera's shittier brother, boasting only a Heavy Stubber for weaponry, yet with a smaller transport capacity than the Chimera. Essentially, the Aurox is meant to be an actual APC whereas the Chimera is more of an IFV, yet GW has forgotten to give it the benefits of an APC.
The Aurox's job description as a light APC is probably a much needed grace for the Imperial Army in the 31st Millennium as the only APCs available comes in ECKS BAWKS HUEG METAL BAWKSES such as the Gorgon Armored Assault Transport and the CRASSUS ARMORED ASSAULT TRANSPORT. Sure there is the Hades Breaching Drill, but that is for a very specific niche. The all-rounder of the Aurox is essential for those who want to transport a large amount of redshirts safely without wasting a lot of points.
The Predator Battle Tank of the Imperial Army. The Carnodon was an extremely versatile tank that could mount a wide variety of weapons. You want it to be a dedicated tank destroyer? You can have two Lascannon sponsons and a twin-linked Lascannon turret for quadruple the fun. You want it to devastate MEQs? Just swap the Lascannons with Autocannons and Volkites and your opponent would roll for Anal circumference as his troops will be facing a barrage of bullets and martian death rays. The only downside is that it doesn't have a hull mounted weapon like the Leman Russ, or a Battle Cannon.
Tl;dr: It's the Malcador to the Aurox's Dracosan or Baby's First Malcador.
The Carnodon is a versatile medium battle tank (or perhaps light, going by armament) which acted as the linchpin for many of the storied successes of the Imperial Army during the Great Crusade. As a premier mobile fire platform, it was regularly adapted to combat a wide variety of foes and respond to any number of battlefield conditions. During the early years of the Great Crusade, it rapidly became a byword for conquest amongst the Expeditionary fleets, a legacy which led to the production of the dedicated Imperial battle tanks of latter-days.
As the Crusade moved ever outwards and supply of the Aurox chassis peaked, it is thought that many Carnodon tanks entered a strategic reserve and were distributed to Crusade muster worlds behind the front lines; with over two hundred thousand examples thought to have been stored pending secondary mobilization in the warrens under the world of Tallarn alone.
So where did they go?
Given how prevalent the Aurox and Carnodon tanks were during the Great Crusade, their apparent complete absence in 40k is particularly noteworthy, as well as without explanation.
The most obvious answer to assume would be that the Adeptus Mechanicus just forgot or lost the STCs for these tanks at some point. However, the problem with that is that there are plenty of far rarer vehicles from the Crusade era that still see use in the 41st Millennium, STC or no STC. In the case of what's supposed to be the standard APC and MBT of the Imperial Army, tanks that are likely mass-produced on dozens of Forge Worlds (and every other planet of Civilized World or greater status), if lost STCs was the only problem with them, it stands to reason that there would still be plenty of surviving Aurox and Carnodon tanks from those days still in service.
Another possible answer is that the Aurox and Carnodon tanks are still being used, just not in the current scope of most stories. Remember the Imperial army was for all intents and purposes a garrison force. IE, they kept the local population from rising up in rebellion until the upgrades brought by the Imperium made them wonder why they fought against the Imperium instead of jumping at the chance to join. Chances are that these vehicles are used by the PDF. By the time the Imperium sends forces to retake or fight for a planet, the stocks of Aurox and Carnodons have been depleted and replaced with Chimera's and Leman Russ tanks.
The other, more likely reason for the disappearance of the Aurox and Carnodon is that they were simply mothballed in favor of the more cost-effective Chimera and Leman Russ respectively. This is confirmed by the Carnodon's 40k rules, with the opening of the Great Rift compelling Mars to recommission the Carnadon STC for mass production once again, although the Aurox was left in the mothball bin. Likely because the more modern Chimera serves the same role while having better armament, transport capacity, and built in vox casters, while the Carnodon can serve as a lighter tank than the standard Russ, which does support the theory that they've been mostly handed down to PDFs since their tanks are generally depicted as being less armed and armored than guard standard.
As to why the Carnodon would be deployed against Chaos, that is probably because Chaos generally lacks significant tank and artillery support but had tons of mooks and daemons. So, a horde of light tanks would be very useful against them.
In the case of the Carnadon, it should be noted that it's not a Battle Tank, it's a Medium Tank (like how the Malcador is a Heavy Tank) or perhaps a light tank by armament being used as a medium tank by role. Thus it was likely replaced due to the Leman Russ acting as a better version of both.