There's upwards of 600 now so choose wisely. Would you bring an adorable companion or an unstoppable beast?! Let's hear it.
Personally, I'd bring Typhlosion. He'd wreck especially with solar burn on!
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I would have to say a special eevee with an infinite amount of evolution stones and a new stone I created "simple stone" which will allow eevee, no matter which eeveelution it is in to revert back to eevee, so then I can have her evolve into any one of her eeveelutions anytime.