I'll go with my most drawn OC and one that I've been having the most fun with lately, Comet Cloudburster.
I came about the idea of wanting an OC by seeing other people making their own characters and I wanted my own. After flipping through a pony generator, I realized that I didn't like the concept of having a standard pony. I wanted to make her more alien and interesting so this led me to wonder what alien traits could I give her while still maintaining that she was a pony. Then it dawned on me to make her a cyborg made by some alien figure.
Appearance: This was honestly just me drawing random pony sections and just drawing parts of it unconventionally. I enjoyed the dual ear design, but what that caused was a difficulty in transitioning the ears to the head in terms of drawing so at first I just gave her some fluff under her ears, but it looked generally terrible, but it gave me the idea to make the antennae that shoot off from her head. That was all well and good, but I also wanted to change her eyes so I originally made them very much alien and inspired by Cteno's eyes by Slugbox. The rest of her body was pretty standard, that is until I hit the tail. I like bulkier tails than fluffy tails and I felt that the tail could very well be a source for uniqueness, so I made it into a prehensile tube... except I didn't want to just end it tapered or rounded. I wanted it to be really unique, so I made it into a flower that slowly was designed to be close to a lotus flower. Then I needed a reason for the flower and so I made it a generator for clouds and a propulsion system for her to break sonic rainboom speeds considerably.
Abilities: These were more centered after I designed Creator, my other OC. His intentions for making her into a cyborg became what her abilities would become. The cloud tail was to be used to hide her presence and to generate an improved acceleration over other ponies so in combat she'd be both stronger and faster. In her most recent of designs, she has sharpened wings and some armor plating.
Personality: I always enjoyed dual personalities so I thought of creating a conflict between her living side and her robot side. You had one side programmed to kill and destroy ponies for the service of the Consortium, and a lift over remnant of her old personality before she died as her biological side that enjoyed making cloud animals and enjoying living. When angered, she can lose control and the robotic part of her brain can take over and self defense mechanisms will kick in. This will either result in her using her tail as a smoke screen and fleeing, or her using her sharp wings to kill without mercy. She will have no recollection of the result of her actions as she is programmed to immediately leave the scene no matter which decision her robotic brain feels is the best course of action.
Why: I made her just for fun. No real purpose other than that.
Other places: She is my current avatar for this website, but she's just fun to draw in general so she gets a bit of showtime on deviant art.