The one Jeff chose ended up being the Kitsune from Japanese mythology. He was primarily intrigued by the shapeshifting and possession powers, the former allowing him to retain his original form if the fox form is undesirable, once he figures out how to use that ability anyway, and the latter seeming like a fun power to mess with people. The fox-fire also seemed like a cool- or, potentially hot- benefit.
He set the time limit for a year, and hit confirm, the Chronivac getting to work. A pain erupted from just above his butt, causing a sharp gasp to escape him as a set of tails, the same brown as his hair, begin to grow. A similar pain overtook his head as his ears began to shift, slowly climbing up the side of his head as fur began to grow on them.
Jeff stood up, stumbling away from his computer so his tails had plenty of room to grow in, the Chronivac already creating a hole in the back of his pants for the two appendages to fit. His sense of balance was thrown off, the twin tails throwing themselves around to try and stabilize him, yet being the cause of the instability. He ends up collapsing on the floor, now-pointed ears twitching at the sound, far more sensitive to his enhanced hearing.