I've always thought of them as being the Legendary Beasts. It's not like we call Articuno, Zapdos and Moltres the Legendary Eagles or anything like that.
For the record, I saw a nintendo document back in the G/S days where one of their officials referred to them as being the Legendary Cats, but whatever. Personally, I've always felt like Raikou is sort of tiger-like, that Entei is very much the lion and that Suicune is sort of wolfish. I have a friend who drew Suicune as a panther, though, back in the day, and it was quite good. ^_^