Americans will spend an estimated $480 million on costumes for their pets this Halloween, more than double what was spent in 2010, according to MarketWatch. Pet costumes make up 15 percent of all Halloween costume sales, with more than 31 million Americans set to buy costumes for their pets this year, up from 28 million last year. Millennials who are 25- to 34-years-old are the most likely to dress up their pets, and often post photos online. Unsurprisingly, Facebook and Instagram appear to be fueling the trend. The top pet costume is a pumpkin, according to the National Retail Federation, followed by a hot dog and a bumble bee.