Okay, here's the deal:
Manaphy can only breed Phione once, but you can breed Phione to make more Phione.
Phione can be bred from its "evolution," Manaphy, but cannot evolve INTO it. This means that they are commonly referred to as two separate Pokemon, rather than a chain.
Phione is not referred to by most devoted fans as a legendary Pokemon. According to tiering, it's only NU.