there's inner mongolia which a province of PRC .But Mongolia is an independant country .
Remember ROC？　Ｒｅｐｕｂｌｉｃ　ｏｆ　ｃｈｉｎａ　？　ROC lost to PRC and fled to taiwan . ROC is currently ruling TAIWAN . ROC still consider itself as the sole representative of china (mainland ,hongkong,macau,taiwan and the whole mongolia ) but has controll over taiwan only ..
when they ruled china they regarded mongolia as a chinese teritory which russia disgreed ..
when PRC defeated ROC (russians as they were communists too helped PRC) , PRC didn't claim mongolia to be a part of china . i think this was a favour returned to russia for their help ..

Mongolia is a sovereign nation