Halloween is going to the dogs. Want to dress up Fifi as Star Wars' Princess Leia? You can do it at Target for $9.99. Is Duke eager to be a super hero? Amazon has a dashing Superman costume for $19.99. "Humanizing accessories," as they're known in the pet trade, are the fastest-growing category in the $41 billion U.S. pet market, according to the American Pet Products Manufacturers Association. About 7 percent of dog owners plan to purchase a Halloween costume for their pet this year, up from 4 percent in 2004, according to the annual national pet owners survey of the pet trade group based in Greenwich, Conn.