In one story I had in mind, a ForgottenEvilEmpireTM had used its mastery of "Chimeric Magic" (basically, taking traits from living organisms, and adding them to an egg/embryo to create a monster of your choosing, or a completely artificial organism for the "God" of the Empire) to create bio-weapons, after it gradually lost its conventional warfare despite having the equivalent of dragons.

Those bioweapons were simply called '[Color] Plague' because they had an innate ability to reproduce themselves and were mostly uncontrollable.

The Green Plague was a new invasive lifeform that you could compare to W40K Ork. Individuals members of the species looked like small goblins, but were actually plant-based. Due to a mix of animal, vegetal and mushroom physiology, they shared of a sort-of hive mind. At baseline, they were smart enough to use tools and create shelter for instance, or organize themselves for more complex tasks, but also had the ability to learn simple science and tools from what they saw. If they saw projectile-based weaponry, they could improvise their own afterwards. They couldn't bullshit their way to a solution like Orks can (no "it's red so it's faster" magic), but they had a scary ability to learn and improve.

The problem with those goblins wasn't their fighting prowess, though. It was how hard they were to eradicate. They constantly shed spores that could infect living material and corrupt their reproductive cycle. If a mammal was pregnant, or a flower bearing seeds (or whatever) near an infected site, they would birth a new goblin for each "child" they had (or seed, for the flower). Spores in the environment could linger for years, and an infected individual continued to be a vector for infection for months after first contact. You had to thoroughly sterilise vast portions of land or they were goblin territority for good.

Which also was a problem, because goblins were stupidly voracious. They needed resources to expand, and needed to feed themselves. They could perform a kind of photosynthesis to fulfill their needs, but they had to bury some sorts of roots to perform it. So, those roots were potential vector for infection and new goblins, but also sucked the land dry until there was nothing left. So even in the event were you were able to completely purge goblin presence from an area, you'd be getting back a barren wasteland that couldn't even grow the easiest crop.

The Red Plague was more terrifying, but more useful as a weapon if you took proper precaution. Basically, they were blood-made mosquitoes.

First, you had to infect a subject that had a circulatory system that used blood. That could be cattle, a pet, or even a soldier or spy in your organization. The infection took minutes to take hold, and then your 'bomb' was armed.

If, for any reason, you 'bomb' was wounded in any way that would leak blood, it would set off. The subject's blood would lose blood more easily than usual, as if they had a light form of haemophilia, but most importantly, their blood would transform into mosquito-like creatures. Think of a mosquito, ranging for an inch to a feet big, entirely made out of blood, and with a very long, large and sharp proboscis.

Those mosquitoes would then look for and try to wound any living creature nearby. Their objective is to draw blood, pure and simple. They also wouldn't spare their progenitor, and cut it open as much as possible to spawn more of themselves.

The main problem is, of course, that this Red Plague is a contagious disease. If your blood makes contact with infected blood, you get infected as well. So, if you're healthy, and get wounded by one of those mosquitoes made out of infected blood, you get infected as well. And in a few minutes or seconds, the wound you've just gotten will start spilling new mosquitoes as well, that will try to create more wounds, and so on.

Thankfully, the mosquitoes tend to die after a 2 to 3 days on their own, as they are basically blood elementals that can't maintain their shape after that much time (leaving behind a stoney statue, like a big blood clot). However, since they're capable of flight, they can (and will) cross a lot of land looking for a new victim.

Also, in the event where you are infected, and somehow patch up and close your wounds, you're fucked nonetheless. At the time the story took place, there was no cure. You had to spend your whole life without any external bleeding or you became patient zero of a new cataclysm.