now its time to feed you a headcanon
ranboos actual name is in ender so no one can pronounce it. the reason everyone calls him ranboo was because one day while he and tommy were out walking around ranboo was trying to practice his english by pointing out things and saying what they were called. he saw a rainbow but forgot exactly how to pronounce it so he pronounced it "ranboo." tommy constantly called him "ranboo" from that point on as a way to tease him, but the name just sort of stuck and now thats what he goes by.