I just noticed that there is a user account in my Ubuntu installation and I can not recall creating it myself.
I understand that Squid is a proxy manager but I don't know why should there be a user account for it!
Squid is not a proxy manager:
Either you or someone else installed squid at some point in time; when you do this, the squid user gets created so that the program can run under that user, which improves security.
If you know you don't need squid, you can remove it, along with all its configuration files and the user by running this on a terminal: