I was training to beat Elsa so I go to Desert Resort and along the way I encounter a Darmanatan,it wasn't shiny but it had things that look like diamonds then the battle begun.(I caught it)
I know exactly what is was.
You encountered one of N's Pokemon, a Pokemon he released into the wild in Black and White. If you activated Memory Link with your B/W game, then N's Pokemon start to appear all over Unova.
That means it was one of N's Pokemon. You can find all the Pokemon N used in Black and White in the area near where you battled him with those Pokemon. For example, you can find a purrlion near Route 1 with those diamonds.
This should clear things up.