Subtitled The Blade of Kringle, this isn't a manga, it's a regular graphic novel, but it's about ninjas, including ninja teddy-bears and a ninja Santa! Erik Craddock's art is great, and the story by Nathaniel Marunas is hilarious.
An aggrieved elf uses a bit of illegal magic to amp up a ninja toy, ordering it to go wreck the toy-building area of Santa's Workshop (a map is included!). The elf plans to come in later and conveniently save the day. Unfortunately, the ninja starts doing his job too well and somehow unleashes a hoard of ninja bears, who go on a wild rampage through the workshop. Only ninja Santa can save the day. Or can he?
This story reminded me very much of My New Fighting Technique is Unstoppable by David Rees, but without the bad language and with better art! I'm thinking mainly of the humor here because it had that same kind of off-the-wall snap to it that made you laugh out loud. I recommend this highly.