Racso is a rat that appears in both of Jane Leslie Conly's sequels to Mrs. Frisby and the Rats of NIMH. He is Jenner's only son, who ran away from home to rejoin the Rats of NIMH to get an education and become someone important. He is short for a rat, has dark gray fur, and is almost never seen without his black beret (which he had found in a garbage can and had formerly belonged to a doll). He is jaunty, self-confident (though perhaps a bit more than necessary), and somewhat rebellious. His best friends are Timothy and Christopher.

His given name is actually Oscar. Racso simply spells it backwards because "Oscar sounds like a sissy, but Racso sounds tough."