Canis Major might represent a number of different hounds. The first is Laelaps (Λαῖλαψ), the dog who was gifted to Eurṓpē (Ευρώπη) by Zeus to protect her from mortal and immortal danger as He hid her away, waiting for Hera to give up the scent of His adultery. The same dog, or a namesake of his, was gifted or passed down to Kephalos, who decided to use the hound to hunt the Teumessian fox, a fox that could never be caught. Laelaps was a dog who would always catch his prey and the fox was a pray that could never be caught. Zeus, stunned by the display, rewarded them both and placed them in the sky.
Another explanation is that the dog was one of Orion's hunting dogs. Orion (Ὠρίων) was a famed hunter who has a lot of mythology to his name. He is also included in a lot of existing mythology. The constellation Canis Major might represent his entire pack or one of them, who are/is eternally pursuing Lepus the Hare or helping Orion fight Taurus the Bull. Aratos, Homer and Hesiod support this theory. In the prime of his life, Orion joined with the Goddess Artemis and Her mother Leto, for an epic hunt in which Orion threatened to kill every beast on Earth. Gaea revolted and sent a giant scorpion to kill Orion. Orion, although very powerful, was overcome by the creature, but Artemis and Leto requested to Zeus he'd be immortalized in the night's sky. The Scorpion was admitted into the heavens as well.
That having been said, Sirius was considered a dog in its own right, and even more so, in early Hellenic mythology Sirius was the second head of a two-headed dog. This dog may be the fabled two-headed guard dog Orthus, who guarded the cattle of Geryon and whom Hēraklēs had to surpass for his tenth labor. Sad to say, Hēraklēs batted Kerberos' brother away pretty easily. As a reward for a valid try, Orthus was placed into the sky.
Canis Major is visible at latitudes between +60° and −90°, and best visible at 21:00 (9 p.m.) during the month of February.