Monolophosaurus jiangi is a theropod from the Middle Jurassic of Xinjiang, China. It's phylogenetic position is rather uncertain - though there will be a monograph coming out soon but I can't remember if there was a phylogenetic analysis associated with this redescription.
Of course the most distinctive feature of Monolophosaurus is the large sagittal crest on the skull. The crest is quite thin and has several openings so is probably not a functional structure.
There was some speculation that the basal tyrannosauroid Guanlong wucaii, also from Xinjiang, represents a juvenile morphology of Monolophosaurus. However intriguing this claim may seem, at least one Guanlong specimen shows signs of arrested growth histologically so is quite likely to be an adult.
Here, you see a fine specimen of Monolophosaurus scoping out a carcass.