I consider anything that is named after a common word rare, especially without numbers or understrokes. (my name is just a nonsense word I liked so l used it.) I have been using name availability checkers, and it seems impossible to get a good name anymore like "Reasoned", "Text", "Chair", etc. without numbers.
I know because I installed the Citizens Plugin on my server and when you create an NPC, you have to name it. The plugin assigns the skin of the NPC to the name you gave it if there is a username with a custom skin.
The player "G" was most likely just someone setting their display name on the server to G. Since I don't believe it was ever allowed to have 1 letter names. Just 2 at the minimum (then letter changed to 3 minimum around late 2011, early 2012). However there are some videos of usernames containing "." (periods) and other odd symbols in the very early stages of minecraft such as the earth video on youtube which is like 6 or 7 years old now.