He names all of his horses “Roach,” which in English, is a very grim-sounding name. It immediately stirs the thought of a cockroach, and it sort of fits the grimy dark fantasy aesthetic that the series has going for it, as well as Geralt’s personality as someone who likes to keep things simple and uncomplicated. He gives the name to every horse he owns, further adding to that.
Meanwhile, in the original Polish version, his horse’s name is Płotka, which to my understanding is a diminutive form of płoć, which is a type of fish, known as a common roach in English, which is actually a pretty small and common fish. It being a diminutive form, it’s also basically saying “Roachie” instead of “Roach,” so all things considered, it’s meant to be a tiny, adorable, cutesy name, but that gets completely lost in the translation. It’s like naming your horse “Guppy” or something.