Fan Regiment: The Hydran Legion
The Hydran Legion are the worst regiment of Imperial Guard in the galaxy, they're also the biggest.
The Hydra System, a series of 4 Hive Worlds, pays 50% of their population to the imperial tithe yearly. Their population is still steadily growing despite this. The secret? These hive worlds have Dark Age technology in the water supply that makes the water highly nutritious and dangerously increases fertility levels. The Adeptus Mechanicus have been trying to study this technology for ages to figure out how to turn it off without harming it, but are yet to solve it.
The Hydran population has very little education, with the average citizen being about as intelligent and a Bone'Ead ogryn. Hydran soldiers are often simply told the basics of how to operate whatever weapon they're handed and pointed in the direction of a problem. This has lead them to become infamous in many sectors for being horrifically bad soldiers, to the point where populations of citizens have committed suicide out of despair when hearing their incoming saviors are Hydran. To counteract this, Hydran soldiers have begun wearing cheap counterfeit uniforms to impersonate better regiments. Nowadays most "Cadian" deployments are actually veteran Hydrans who proved their mettle by not dying in 2 or more battles. This way the valuable resource that is Cadians can be used more surgically.
The biggest battle The Hydran Legion ever fought was against the Tyranids for 50 long fought years. The Hive's population still managing to outpace the swarm's consumption, helping keep the monsters at bay. Finally, victory was declared when the Tyranids had their fill, and left the system before finishing their meal. This was considered the greatest military victory in the history of the Legion.
At this point I hope it's clear that this Regiment was invented as a joke, just a joke I've been lovingly developing for a while. The Hydran Legion is meant as a sort of silly explanation for when guards in the tabletop/lore get curbstomped a little too hard even for them, it was actually Hydrans in disguise.
Hope some folk find this funny.