What kind of a furry body does John choose for himself?

A trappy femboi fox.

While John was still primarily motivated by a need to breed

He wasn't totally sold on the idea of being on top of the pack. He'd much rather just have a family that were remarkably easy himself included.

The change hit him fast, a bit of the muscle mass he had snapped into existence transforming into more

untrained tissue, his tight rear began to shift bulging out and softening. After a few moments it was transforming into a pleasing and plump bubble butt. He lost a bit of height becoming barely taller than Melanie as an orange and gold sheen began to weave its way through his skin. He felt his balls plump and swell with fertile fox seed but he also felt his desire shift. He didn't just want to take his sister's, he wanted their dad to take him. He couldn't get pregnant but it was definitely going to enjoy his new body to the fullest. His tongue lolled out licking around his lips as they became fine and angular pushing out of his face until mouth and nose became one pushing a fine canine muzzle out past his eyes.

He groaned finally realising that Melanie was still teasing the end of his shaft as it became pointed just like the creature he was emulating. He panted heavily something in his transformation had evidently given him a hair trigger and the power to recover as he sprayed semen all over Erica's face relishing in the change as his long golden red tail burst from the base of his spine.