I heard you can catch Phione in Diamond, but I am not sure. Is it true? If it is, where and how do I catch it? Please help me! Thank you! :)
Note: Horrible Speller, so sorry for any grammer or spelling mistakes!
No, it is not possible to catch phione, as it is the offspring of Manaphy.
You must breed Manaphy with Ditto to get Phione.
Source: Experience and this.