I'm going to claim this as my critter for today even though it has been gaining attention for other reasons. This is Similicaudipteryx which is roughly 125 Ma old and was found, predictably for a feathered dinosaur, in China. Although it is a theropod it appears to be a herbivore, with robust jaws and buck teeth. The two fossils found are of a juvenile and an older specimen, allowing the ontogeny to be compared.
What is surprising is that the feathers differ markedly between the two. This is not too unusual, considering birds go through two stages of change in feather type, from downy feathers in youth to adult plumage. The juvenile displays ribbon-like feathers mixed with down, whereas the older dinosaur displays longer quills and down. It is thought that feathered dinosaurs start with only down for insulation, meaning these dinosaurs may have gone through three stages.
The ribbon feathers of the juvenile are even more perplexing as they superficially resemble display feathers, such as those found in birds of paradise. They would have provided no benefit for insulation, and ornamental features normally appear during sexual maturity. What use is display in a sexually immature juvenile? The authors of the paper appear to overlook the possibility of it being used as a visual deterrent to larger predators, though why they are not retained in adulthood would be baffling.
These unique feathers have been lost in time, they only superficially resemble display feathers. It seems that the dinosaurs had a greater diversity of feathers than modern birds. Perhaps there was more flexibility in the development of early feathers allowing for a greater diversity which was later cut down and channelled.
The paper can be found here: http://www.nature.com/nature/journal/v464/n7293/full/nature08965.html