Ogey Rrat came from Pekora's MGS3 playthrough where she was responding to chats and said Okay which sounds like a retarded Ogey. Then she sees an item with a R. Rat written above and read it wrong.
It became very prominent in /jp/ as a funny meme usually posted alongside enlarged pekora images. Sometime before the creation of /vt/, it became an actual Rrat with rat images. Then it evolved once more in /vt/ as shitposting mainly for schizo posting.
https://youtu.be/IuwYZYlJoPY 