These are custom armies created by fans of the Warhammer 40,000 tabletop game. Specifically the Adeptus Astartes or more commonly known as the Space Marines. Some of these armies (or "chapters") parody, mock and/or are making fun of certain aspects of the Space Marines or the game and/or game's setting as a whole itself. Others possess random or bizarre characteristics.
4chan, specifically the /tg/ board of traditional gaming (which concerns non-video game games like board games, tabletop games, RPGs, etc.), has made several custom factions and armies of their own through out the years for the tabletop game Warhammer 40,000, most notably the Adeptus Astartes or commonly known as the Space Marines; Genetically enhanced supersoldiers of the Imperium of Man.
Space Marine Chapters
There has been a lot of custom Adeptus Astartes chapters that have gained popularity within the WH40K fan communities. These are just some of the most earliest and/or most notable custom chapters:
The Angry Marines are the most parodied form of Space Marine. So much so that an entire fanmade codex was even created specifically for them. They are characterized by their constant state of rage and anger, their shouting and screaming swearword ridden speech, and their brutal and unorthodox method of fighting. Their battlecry is "ALWAYS ANGRY! ALL THE TIME!". Their color scheme is primarily yellow with red secondary color or trim. Their insignia is either a red or yellow angry face emoticon smiley or >:(.
They are almost exclusively a melee combat oriented army utilizing a Kamikaze style of deployment where soldiers and vehicles are launched directly at their opponent. Whatever mission they are deployed, it always end up not just the opponent's destruction but also everything else.
Their armory includes a myriad of extremely exaggerated versions of normal WH40K wargears such as the 'power feet' which are a parody of the 'power fist' which they use along side the former. They do use ranged weaponry but most of which have been converted from traditional ones so that a space marine or a vehicle may be used as living ammunition and instant deployment launchers such as the 'Angry Marine Launcher' which, as the name implies, launches Angry Marine into combat and the 'Angrinator' which is basically a predator tank with several Angry Marine Launchers mounted on it.
Another fairly well-known chapter, the Pretty Marines are categorized by looking like a Japanese Bishonen warrior, who remain impeccably beautiful despite getting into horribly disfiguring combat. The main gag being that the actual Space Marine chapters are nowhere near as pretty-looking, both because of said disfiguring combat, and because the massive amounts of genetic modifications that the Space Marines have render them less charming in appearance.
Pretty Marines are often times considered to be at-odds with the Angry Marines, though the reasoning behind this is currently unknown. Potentially, it could have something to do with their looks.
The Reasonable Marines are a chapter that "are what Space Marines would be if 40K was less grimdark, the Imperium was a benevolent governmental body, Tau were happy Koreans and Abaddon was competent." Instead of wearing incredibly obvious colors on their armor, the Reasonable Marines use camouflage to sneak up on enemies, make heavy use of tactics involving cover, and are very practical. The main gag, of course, being that the actual Space Marine chapters are nowhere near that careful and prefer to charge at the enemy guns blazing.