The 'dex says: "Volcanoes erupt when it barks. Unable to restrain its extreme power, it races headlong around the land." (Silver) "A Pokémon that races across the land. It is said that one is born every time a new volcano appears." (Gold)
Why that doesn't make sense: Technically, we need two Pokédex entries to really figure this guy out. The first, from Silver, says that every time Entei barks, it causes volcanoes to erupt. That's not really possible, but makes more sense than the second entry, which says that every time a new volcano appears, an Entei is born. But… Entei are Legendary Pokémon, meaning they're super rare (many Legendaries are the only of their kind). Also, doesn't this mean that an Entei's bark could sprout a new volcano, which erupts to create another Entei, which will eventually bark, which sprouts...oh god… the second a car drives by, the world would be absolutely covered in lava.