To be honest I don't know much about Dinky. Drawing her here was sort of practice since I got a request to put her in a future picture. From what I understand she was just a random background pony that happens to have similar colors and mane style to Derpy. And it wasn't helped that she was seen in close proximity to Derpy. So a group of fans decided to name her Dinky and that she was apparently the daughter of Derpy (despite one being a pegasus and the other a unicorn)
I just finished reading it and I have to say I loved the story. Though the writer really should have seen an editor. Also I'm of the crowd that thinks Derpy is just special and not retarded, I feel weird looking at the character in that way or maybe retarded is to blunt of a word.