Wheel. Wheel and tyre 75mm wide x 235mm diameter. Suits  P0475H 48 mm ribbed / serated 48mm with slimline top Jockey wheels with Bradley HU12, Knott KFG35, KFG27 couplings or the P0209 clamp

Some people also call them dolly wheels, trailer mover, trailer hitch wheel, caster, jack, support leg.