They're from scratch. In this case, I decided that I was bored and I wanted to pixel something. I followed that decision by deciding I wanted to challenge myself, and to use no references at all. So, I came up with the pattern, shading, and design using only what I remembered from my history with pixel art.
I should also note that every single one of these is an indexed .gif file that contains a palette of 8 colors. None of them are over one kilobyte in size, and I'm fairly certain they're all under half of a kilobyte.