Is it haram to use fake names (especially online) ? Most of us in this website do so.What about using another person's name ?
Basically the matter is neither clearly black nor white (See for example this comment).
Most scholars who consider this haram are having in mind that some people may use wrong names (not usernames/ID's) so if you call yourself: Ahmad ibn Hanbal and you are not called Ahmad nor your father's name was Hanbal this can be regarded as naming oneself after a different person than the own father which is rather sinful and haram and it is a lie (also sinful/haram). And Allah says:
Call them by [the names of] their fathers; it is more just in the sight of Allah (33:5)
While a username is a totally different matter, as anybody using the plattform knows that this name is not necessarily your real name.
On the other hand it is the right of a person to introduce oneself by his/her real name or to keep oneself's identity hidden for safety reasons or any reasons that have kinds of benefits that a human may require (in order to avoid being harmed).
It is even allowed to add wrong attributes etc. and using other person's name according to (this fatwa #63095 on islamweb) as this by default isn't lying as it was defined by scholars:
، فالكذب كما عرفه أهل العلم هو الإخبار بالشيء بخلاف ما هو عليه على وجه العلم والتعمد، واستعارة الإنسان لنفسه اسم المقاتل لا يقتضي أنه يخبر الناس بأنه مقاتل، وإنما هو تعريف بنفسه لا غير.
Lying as defined by people of knowledge is informing (others) with information that differs from the real (matter) by knowledge, and intentionally. And using (for example) the name of a fighter doesn't mean by default that one informs the people that one is a fighter, but one simply introduces oneself as such.
This sounds a bit weird and self-contradictory therefore I try to explain how I understood the matter by giving yourself the name TheMadHatter or Superman or Medi1Saif you simply introduced yourself, but didn't tell people I'm called TheMadHatter, and they further could ask you about your real name if they want. Only if you then lied you've committed the sin of lying not earlier!