Riolu pants, a look of utter content on his muzzle. He smiles as he turns his gaze down to his groin and the friend contained within, "Human... That was absolutely incredible. Thank you. Let's get you out of there, though. I don't want to leave you in there while we're travelling."
You feel a moment of disappointment at not being allowed to stay longer. Then again, it's up to Riolu and the aura dog wants you out. The least you could do in thanks for the good time is not freeload.
Fortunately, you get to stay a little longer as Riolu rests and recovers. Time you spend enjoying and pampering the dog's interior. Which, if the gentle moans and the smacking of a wagging tail hitting stuff says anything, was doing a good job pleasing and keeping Riolu worked up. Riolu soon joins you in attending to his genitals.
In almost no time at all, Riolu find himself teetering on the edge. With another howl, the aura dog reaches his second climax. For you, his testicles shift, even churn, then tighten. With a rather aggressive squeeze, the balls force his cum upwards.
They force you upwards as well, except you are promptly halted. Unlike with Braixen, the sphincter that separated Riolu's balls from the rest of his genitals was tight. It allowed semen through, and no more. It had let you in because it was forced to by the muscles behind it. But the aura dog's sack was far from being as strong.
When Riolu's climax finally died down, you remained inside your partner and friend. You look around for a moment, then feel your friend shift, "Where... Is he?" You press into the fleshy wall, prompting the dog to yelp, "W-What?! He's still in there? Human! Are you okay?"
You are... Trapped. It hadn't worked, getting you out. Riolu's body was... It seemed as if it was too small, too tight, to release you. Riolu must have been sensing your aura, as he reacts to your thoughts.
"Braixen! He's stuck. He thinks my body is too small," Riolu sounds worried, panicked, and even... Regretful. No wonder. The Pokemon may very well have become your prison, "W-We need to do something!"
"Well... If this is happening because you're too small, then you just need to get bigger," Braixen nods to himself, "Yeah. You'd become bigger after evolving, right? So we just need you to evolve."
Riolu hesitates, then responds