[@zarkun] Well according to this Forum:

[url]http://www.serebiiforums.com/showthread.php?337035-the-3-legendary-dogs-cats[/url]

I'd say dogs. Entei's barks cause new volcanoes to sprout up :)

//edit: scrolled down some more. Maybe just settle for beasts xD That's what Nintendo officially knows them by. Also, the tiger/lion/leopard idea is pretty convincing, esp for Raikou, although I'd contest that Entei/Suicune look more like dogs.