Chickenosaurus is a hypothetical creature that Jack Horner wants to create. It is a chicken with a tail, hands and teeth. Horner described his project in his 2009 book How to Build a Dinosaur: Extinction Doesn't Have to Be Forever. The project was featured in the documentary films Jurassic Park: The True Story and Dinosaurs: Return To Life?.
According to the scientific consensus, birds are dinosaurs. They retain genes from their ancestors that can make chickens develop arms, long tails, and even the beginnings of teeth, but these genes are turned off early in development. Horner is planning to stop the genes from being turned off, making a chicken with ancestral dinosaur features.
Ancient birds, like Archaeopteryx, had a long reptilian tail. Modern birds still have a vestigial tail. The main focus of Horner's project is to identify the gene(s) that cause inhibition of tail development in the embryonic stage. By changing the expression patterns of these genes, chickens with long dinosaurian tails could be bred.
The idea has attracted criticism from many scientists. Sean Carroll, a developmental biologist, has experimented with the development of insects, and these experiments have usually killed them.
Matthew Harris, a scientists who caused rudimentary teeth to develop in a chicken embryo, shares the same skepticism, saying that there are no longer any genes for dentin or enamel in the chicken genome. 
The idea to create a chickenosaurus existed for a long time. Jack Horner proposed the idea at multiple laboratories. He wrote that the people liked his idea, but never had the resources to start such a project. When Larsson and Horner started the project, Horner donated 20.000 dollars to fund a postdoc for one year in Larssons lab. Since then the project recieved many private donations, including some from “Star Wars” filmmaker George Lucas.