[... It's funny, when Athena first arrived, she'd swear to you that she wouldn't get attached to these animals. That they were all just time bombs, waiting to turn into monsters that could put the Pandoran wildlife to shame.

But now... now, she can't stop that flash of pride as Ash looks so awestruck.]

This is Cerberus, an Arcanine. [Cue happy bark from the big doggo!] You can find his previous evolution to the west of Ecruteak City. To allow it to evolve into the form you see here, you need an item called a Fire Stone.