Its a scythe weasel, its possibly the most badass summon in the series so far. The scythe weasel is the asian cousin to the artic ice weasel. I thought Kamaitachi was an old legend having to do with an old man in a mountain. Also refering to a mountain region of japan were the winds are so fierce that they are said to cut things. But hell i'd rather it being an eye patch'd ferret wielding a scythe with dozens of spinning kama around him. Too bad the kama weren't kusarigami.....

As a side note i think the term "Itachi" is jap. slang for a burly lesbian isn't it?