Network in Finder Sidebar

Typically this is some device on the local network. Depending on where you are (or even where you were) the Finder may show you other devices on the network that belong to others.

Without knowing more about your network topology it's impossible to be more specific, but consider options - for example if you're in a local coffee shop you may see other customers' systems on the network.

If you're on a private wireless network (e.g. your home) then it may be possible that someone is piggy-backing on your network. Check your router settings to see if it has a log of connected devices, and change your WiFi security/password if you're worried.

That's normal. Especially if you ever turned on file sharing on your Mac. If it was ever on an active directory network, or with a Microsoft account, it is possible it got itself assigned a computer name. Not a pretty one, but there it is.

If you ever took it to a public wifi, it might have picked up on someone else's file share as well. But that computer would no longer be accessible, unless the two of you went back to the same public wifi and the settings remained the same.