Elena was always a switch but earlier in the year had found a Mistress and accepted her collar; serving as a slavegirl is everything she's ever wanted, worshipping a Goddess, being bound and played with, serving to make her owner's life easier and even being rewarded with the occasional orvasm. Every so often though she feels those parallel urges creep in and just wants to tease a boy until he cries.
At first she managed to keep it under control but one night at a BDSM party she was found edging a bound slaveboy and feeding his precum back to him. Elena was punished for touching someone else's property without permission, everyone at the party gathered to watch her ass be caned raw, but her Mistress knew the underlying cause.
Luckily slaveboys were easy enough to come across and within a month she had a well trained puppy on his way. It would be Elena's job to feed and walk him, deliver his daily dozen edges and then lock him in the crate at night, but she was granted full rights to tease and torment the new pet however much she wanted as long as he was never, ever, allowed to cum.
To ensure her girl never took pity or got careless Mistress created a new rule that would affect both her slaves; if Elena ever got careless and let the puppy orgasm she would take its place as the house pet for the slaveboy to use however he liked. She would remain a denied puppygirl until he fucked up and accidentally let her cum and they'd swap again.
It was a whole year before it finally happened, she was on edge #11 and miscalculated how much more he could take leading to a puddle of cum pouring out. She tried to hide it, made the puppy lick it clean and edged him another dozen times to get him backed up again, but there was no faking the level of frustration he was meant to be enduring and the next morning she was locked into a matching bitchsuit before the slaveboy began her dozen edges.