Big Hero 6 went through a number of changes before becoming the superhero movie we've fallen in love with. And one of the things that was ultimately cut from the film was Hiro building a set of rocket boots for his cat, Mochi.
When we visited Walt Disney Animation Studios, we learned that, for a while in the film's development, Mochi appeared at various points in rocket boots. Apparently, the Japanese marketing department loved Rocket Cat so much that he continued to appear in promotional materials even after he was cut from the film. And this concept image by visual development artist Mingjue Helen Chen shows us a not terribly pleased Mochi in his boosters. Incidentally, Chen's artwork is consistently delightful; you can see more of her Big Hero 6 work at Concept Art World and I highly recommend following her on Tumblr.
A nod to Rocket Cat does appear in the final film, however. You may have even caught it in the trailer.
Big Hero 6 Concept Art by Mingjue Helen Chen [Concept Art World]