It's not so much I'm telling you that you need to keep the character. The concept of the contest is to draw an OC, existing or otherwise, in the theme of a video game. We aren't telling you to create a whole new character, you could just be drawing a pony with a theme! By no means are we telling you to make a character that you won't keep.
Entering a contest with the mindset that you won't win isn't appealing. Many people make characters and sell them afterward because they didn't win. If you'd sold the adoptable before the contest, wouldn't that just make it an adoptable?
Bottom line, this isn't a character design contest. It's a "draw a picture with a theme" contest. It's not a valid entry because you made it with the intent to sell it, not the intent to make an entry for this contest.