Why does the canis majoris called the canis marjoris?

"Canis Major" is the Latin name used for the constellation "the Greater Dog".

The main stars have Greek letters in the Bayer system, combined with the constellation name.

When naming a star in a constellation, the genitive (possessive) case of the Latin constellation name is used. For Canis Major that is "Canis Majoris".

For example, Sirius is Alpha Canis Majoris in the Bayer star naming system. It means "Star A of the greater dog".

(Sirius is also called the 'Dog Star'.)