I never saw Arcanine as very dog like, always looked like a tiger lion thing to me. As it turns out it's based on shisa and fuu dogs. My Arcanine is one part lion, one part tiger, with a sprinkle of bear, and a dash of dog. Now, onto the flavor text:

Arcanine is the mightiest of all fire types. There are tales of great fire dragons and giant birds lit aflame, but Arcanine is the most powerful confirmed to exist. They have garnered their nickname as the "Legendary Pokemon" as they are extremely rare in the wild and have only recently have specimens been evolved in captivity. The the key to the the development of an Arcanine is that the juvenile state, Growlithe, needs to ingest volcanic soil. In the wild Growlithes will unknowingly consume volcanic soil through ingesting gastroliths from the stomachs of Torkoals. No one is quite sure why a Growlithe will under take an extreme growth after this, though it has been theorized that it is a defensive response to other large Pokemon in the area. In captivity one need simply grind up 1/4 cup of volcanic gastroliths(fire stones) into powder and mix it into the Growlithe's diet. Arcanine and their kin are exceptionally fierce, but playful Pokemon. Young Growlithes are often seen taunting other Pokemon, even ones much larger than themselves. Turtwigs are notoriously sensitive and the playful jaunts of a young Growlithe will make them stupid baby cry.