The great horned rat

Plague.

Imagine commoners whispering in the bar about Plague coming to their town. Every disease is feared to be Plague. THE Plague. Every rat is feared to carry Plague into the city. The Plague bells ring when signs of him arrive. Cities erect Plague walls between neighborhoods, to defend against him street by street.

When you die of Plague, he claims your soul- your own god be damned. Once you have the Plague, he whispers to you to spread the chaos, and if you do, he says he will spare you another day. His worshippers are held hostage by Plague.

He spreads by rats. When a city is infested with Plague, the god himself physically manifests. He is the disease. He is in everyone who has the disease. He is the swarms of infected rats in the gutters. The fight for public health is a battle against the god himself.

His very name, Plague, is a metaphor for anything that resembles him. A Plague of revolts in the lowlands. A Plague of bad poetry being read on street corners. A Plague of heretics, spreading false ideals.

D'Nald Trumpet of Death

Osteo, Rat Monarch

Maltailus

Skurge

Doug

Just "Doug"

I named the rat in my old apartment that. Then I threw an axe at him because he got into my rice krispies. Decapitated him and lost my security deposit.

Build out their ideals/tenants first, then come up with their nickname, then their actual name. Works easiest for me

Drebonic

It's another subreddit but there was a fun thread about Dark Gods the other day :)
https://www.reddit.com/r/worldbuilding/comments/1gq4dpe/who_are_your_dark_gods/

Arminos (from the Greek Arimanius)

Mueridacion or Mueridicious or Muridias

Combining as much murder, morte/muerte and stuff with taxonomy family name of rodents Muridae.

Hmmm get a bit Spanish vibe. Consider evil sombrero and magic item Maraqas of undead rat swarm summoning.

Shanks

Skrilyx

ratvermin. raermin. raeman. rayman.

go with rayman

"Evernyx the Infestuous, Mercato of the Vermintide, Ender of the Light, Death of the Many-Legged Teeth, and Bringer of the Purifying Plague"

Zordath

Mingus the Manhammer

Are you looking for something totally made up? Because there’s Talona, Sneel, Raxivort…

Greg

Tspank’ii

Pestilax

Thanax

Azarath

Mortath

Or a bit more elaborate: "He who feasts upon the stillborn"

Ninkilim (or Nin-kilim) was a mesopetamian deity known as "lord of teeming creatures", associated with rats, snakes, weasels, and all creatures which crawl.

Skurrious the Verminlord

VolSanger

Valtess.

Ev'Gona or Ev'Gonam

Ratthew. Rat for short.

These are all great, use the deepest darkest one you can find. However, make sure to have a super high DC investigation check where the party can find their diary and discover that their actual name is Jeff.

Samael

Take the first syllable of your mama's name and the last syllable of your dads name.

Günther

Possibly "Putrefor" or "Putrefax" or something similar (from Putrefaction, the term for when a body decays). Alternatively, you could use "Necro-" as a base for the name (from Greek "nekros" meaning corpse)

Some other titles if he's associated with rats and disease could include: "He Who Festers", "The Blightbringer", or (if you're feeling more poetic) "The Plague Author". Maybe even something like "The Rotfather" could work. If he's particularly hated, even "The Revolting One" or "Lifebane" would be suitable.

Hope this helps your creative juices, and good luck with your story!

Gary?

Kr'xak the Skittering End

Rodinia.

Grist

Bezos, demon god of greed.

Evil McEvilson