grouch — to be sulky or morose; show discontent; complain, especially in an irritable way.
complainer — A complainer is someone who complains a lot about their problems or about things they do not like.
malcontent — not satisfied or content with currently prevailing conditions or circumstances.
whiner — to utter a low, usually nasal, complaining cry or sound, as from uneasiness, discontent, peevishness, etc.: The puppies were whining from hunger.
groaner — a low, mournful sound uttered in pain or grief: the groans of dying soldiers.
bear — If you bear something somewhere, you carry it there or take it there.
belly-aching — Informal. a pain in the abdomen or bowels.
bug — A bug is an insect or similar small creature.
crab — A crab is a sea creature with a flat round body covered by a shell, and five pairs of legs with large claws on the front pair. Crabs usually move sideways.
crank — If you call someone a crank, you think their ideas or behaviour are strange.