Roaming and Delta Pokemon

I found Delta Clamperl and when I engaged it Entei was coincidentally in the area and I encountered it instead, this isn't too much of a problem as I save before engaging Delta Pokemon, if I hadn't though I would have missed my chance at getting Delta Clamperl. I tried catching Entei just to be sure that Clamperl would disappear afterwards and it did.

I'm on version 1.2.2