How do you obtain one?!

I believe you get it when you capture 101 arkadions

You get Arky when you have 101 Arkadions. Note that you don’t need to catch them, only have had them in your team at some point. So, say, thunderback counts as 4 Arkadions because it evolves 3 times and you’ve had each evolution. Arky evolves into Arkwjng, by the way.