What does "companion + [noun]" mean? Is it a common expression?

Native speaker’s answer
Rebecca
Yes, "companion" is a noun, so it's not usually used as a modifier. It does sound a little weird here. "Companion" usually used to refer to people, or things that complement each other. "Dog" is more natural as a modifier, so you can say "dog companion" instead of "companion dog." Ex: My best friend is my travelling companion. Ex: I take Max, my dog companion, everywhere with me.