Testudo
The Testudo was an armoured personnel carrier (APC) that was used by the Imperial Army during the Great Crusade and the Horus Heresy. The Testudo was the Chimera's predecessor and apparently looked no different than today's APCs. The reason why the Testudo was phased out by the Chimera was because the Testudo was lightly armed and armored whereas the Chimera have the same function as the Testudo but with improved defenses and armoring. Another disadvantage of the Testudo was that it used solid rubber tires, which were less dependable than metal tracks later used by the Chimera which is stupid because replacing a flat tire takes less time and is so much more easier than replacing a destroyed track. Its disadvantage against the Chimera is also bizarre since the Testudo is one of the few Imperial vehicles that actually has sloped armor (If taken that its design is a modern APC) whereas the Chimera doesn't, meaning that the Testudo has more chance of surviving a hit from a anti-tank weapon due to its sloped armor ricocheting the shot while the Chimera has to eat a direct hit. Another example of GeeDubs failing again. The vehicle's main advantage over most other military vehicles of the time was its speed, as the absence of thick armor or heavy metal tracks allowed the vehicle to reach impressive speeds.
The weird part of the Testudo is that despite being lightly armored, it somehow weighed at a hefty 40 tons which is as heavy as a modern Main Battle Tank. Whether the Testudo was lightly armored in comparison to its weight or it was another example of developer oversight doing a typo is unknown. The Testudo is also one of the few wheeled vehicles of the Imperium, the other being the Tauros.
Alternative viewpoint
The tracked vs wheeled vehicle debate is a very long and storied one in military design circles but as a rule of thumb, the heavier a military vehicle is, or the more it's expected to go on rough or soft terrain, the more it benefits from tracks. Similarly, tracked and wheeled vehicles have very different turning mechanisms, with tracked vehicles generally being more capable of turning in place than their wheeled counterparts. Wheeled vehicles certainly have their advantages, but if there were no advantage to using tracks then nobody would use them today. As for sloped armour, modern high energy projectiles like APFSDS rounds would not ricochet off of the armour they failed to penetrate, as there's simply too much energy involved; instead, they instead shatter. So while sloped armour used to increase the chances of deflection, now its primary advantage is using geometry to add to the amount of material the projectile has to go through without using a thicker (and thus more expensive) plate. Sloped armour does have a cost, however, namely in the form of interior space, which is why generally modern tanks no longer have sloped side or rear armour like some World War Two or early Cold War vehicles did. Similarly, the Testudo's forty-ton weight isn't that far out of the bounds of modern Infantry Fighting Vehicles (IFVs), particularly the T-15 Armada platform, though notably most of the heavier IFVs of today are tracked. Neither the Testudo nor the Chimera are particularly bad designs for an IFV; certainly there have been more hare-brained schematics proposed in real life and much weirder shit in the setting, but given the Imperium's frequent need to deploy on backwater planets or in poor terrain for wheeled vehicles one can see why they'd switch to the tracked Chimera as their IFV of choice, and in the hyper-lethal battlefields of the far future where you'll never be able to dodge every shot with the speeds that the Guard's vehicles can go, stronger armour plating probably would save more lives than simply going a bit faster.
Weaponry
The Testudo was armed with an Autocannon mounted on a small turret, which was capable of turning a full 360 degrees, which allowed the vehicle to engage enemy units whether it was travelling towards them or fleeing. The Testudo's troop compartment was capable of carrying an entire infantry squad, up to twelve soldiers, yet lacked the Lasgun mounts featured on the Chimera. The troop compartment only had a single exit, a rear ramp wide enough to allow two soldiers to exit the vehicle side-by-side. The inside of the troop hold featured benches that ran the length of the hold, allowing the passengers to sit down. Near the front of the passenger hold were ammunition baskets that held the ammunition for the vehicle's turret-mounted Autocannon. It is unknown if the Testudo was able to be outfitted with other weapons or upgrades.