Silly, rather creepy Red Sox mascots "Lefty" and "Righty" (aka Left Sock and Right Sock) with Wally the Green Monster.
I haven't seen the Sock mascots at Fenway for several years now... Maybe they're retired.
Wally is a big Muppet-like creature who is supposed to represent the 37-foot 2-inch wall in left field -- the Green Monster. The (real) Green Monster is in the background of the three mascots.
I'm not a big Wally fan, but the kids like him...