Monomer is a term used to define a single unit or molecule that can react with another molecule to form a repeating fashion of large compounds known as polymers. Carbohydrates are generally defined as group of organic compounds which mainly composed of carbon, hydrogen and oxygen.
Carbohydrates indeed have monomers. Examples of which are glucose, galactose, fructose, etc... Basically they combine together in order to form polymers of carbohydrates.
More importantly, carbohydrate molecules have its general terms. For the single unit, it is called monosaccharide (similar to monomers), for two molecules it is disaccharide and so on. Polymers of carbohydrates are called polysaccharides.
Monosaccharide exists in two forms: straight chain and ring form molecules. However when they bind together to form polymers, the ring or cyclic structure is considered.
Monosaccharide – glucose; galactose
Disaccharide - Lactose (glucose + galactose); glucose (glucose + fructose)
Polysaccharide - Starch (repeating glucose units)