Hmm, I was going to get a screenshot of the automatic name suggestion thing, but I'm not seeing it on the create account option anymore (on Android app or Web).
Anyway, the takeaway is there is a way to play on the site, that creates those usernames when it's used. It's all legitimate and apparently the site is using the old suggested name process to create the accounts, along with the letter_ prepend. I believe those are tied specifically to a person, in a similar way people can sign up and link with Google, Facebook, or Apple ID but automatically creates the username.
Maybe they are all members if the same family and that is the way they detect if they are playing vs a cousin or a brother.
Martin literally explained it. These are the accounts created by the chess.com system when you play as a guest.
That's not exactly what I said. I said they are from a way the site has to play chess that is similar to how the site handles guests. They aren't guests but are still real accounts tied to people.
The usernames are just ones that the site normally has as possible names when a new account is created with the prepend of the letter and underscore being used by that site access process when creating the accounts.