Rat Ogres are one of the most successful and special breed of mutant monstrosities cooked up by Clan Moulder, and the only one to consecutively receive rules support throughout multiple editions. They are a twisted, mutant species that resemble ordinary Skaven bulked up into giant, hulking versions of themselves - hence the name "Rat Ogres". It's unclear if they are some twisted hybrid of rat and Ogre, or if they are a strain of Skaven mutated through warpstone-based steroids and other unholy concoctions.
Rat Ogres are prone to all manner of twisted mutations and surgical implants. They are virtually mindless, but this combination of unnatural loyalty (an unheard of commodity amongst Skaven) and sheer killing power makes them greatly sought after by Skaven. Powerful warlords and Grey Seers love to have Rat Ogre bodyguards, and they are one of the deadliest field troops that Skaven can have, making them the most popular and common of Clan Moulder's creations.
There was a short-lived gaiden game in which the players took on the role of aspiring Mutators, commanding stables of Rat Ogres and other Clan Moulder beasties in pit fighting scenarios to achieve wealth and fame.
But Skaven always want something better. With The End Times, Clan Moulder has finally surpassed the Rat Ogre with the creation of Stormfiends. Rat Ogres have always been stupid, and Throt the Unclean had learned that smarter Rat Ogres became less aggressive. Initially, he tried stitching Packmasters directly onto the beasts, but the end results invariably went out of control as the pissed off Packmaster expressed its frustration via the Rat Ogre it was stuck on.
Throt soon figured out a way around this: by mutating a will-broken Skaven slave to have a disproportionately large brain and physically grafting it to a Rat Ogre's body as a secondary brain, Clan Moulder created smarter yet still loyal Rat Ogres. Though no more lethal than the standard form on their own, a fortuitous alliance with Clan Skryre allowed then to be outfitted with heavy armor and a variety of surgically implanted weapons: ratling guns (modified with a cut-off mechanism, as neither the ogre nor its brain-slave had the self control to not fire all the bullets at once), warpfire gauntlets, and windlaunchers let them devastate formations from afar, while doom-flayer gauntlets, grinderfists, and shock gauntlets granted them far greater melee strength than an ordinary Rat Ogre.
The closest thing to rat ogre special characters in canon are Boneripper, the eternally worfed bodyguard of Grey Seer Thanquol, and Ghoritch, who is actually a Khorne Berserker whose brain was implanted in a rat ogre body.