I heard a scholar in Tunisa on YouTube say our beloved prophet Muhammad (saw) real name Quthem?! Also Abu Kabsha another name?! Is there any truth to this claim from any historical reliable sources?!
Muhammad and the name Qutham
There's a story saying that one of the son''s of 'abd al Muttalib who died while Amina was pregnant was called Qutham قُثَمُ and maybe he wanted to name his grand son this name but a revelation came to Amina and him telling them to name him Mohammad (see also Why did Amina never remarry?).
Note that orientalists like Nöldeke, Springer, Derenbourg and Caetani came with the theory that Muhammad's name was Qutham and he later changed it to Muhammad as the name Qutham was not quoted in the Torah and New Testament.
Note that this name القُثَمُ in Arabic has a good meaning (see for example in al-maany): like the generous, the assembler of good stature and manner.
Muhammad and ibnu abi Kabshah
أول من عبده أبو كبشة أحد أجداد النبي صلى الله عليه وسلم من قبل أمهاته
That's why the disbelievers used to call him ibn abi Kabshah (the son of abu Kabshah), because he rejected there believe and have choosen an other one like his abcestor. This is stated in the sahih narration where ibn Abbas is quoting abu Sufyan (before his conversion to Islam). Al-Qurtoby also quoted this expression of Quraish:
وقالوا : ما لقينا من ابن أبي كبشة
Note that Arabs when they want to humiliate somebody they ascribe this person to an unknown ancestor and an ancestor from the mothers line (Name of the father of the mothers mother and so on) is an unknown person. While if they want to praise a person they ascribe him to a known ancestor!
Note that the Prophet even had a servant called abu Kabshah who died during the Khilafha of 'omar ibn al-Khattab 13. a.H.
So none of these names are the real name of Muhammad, but names some of his enemies used either to avoid calling him by name or to humiliate him by using a nickname.
Our prophet himself taught us his names in this sahih hadith. Al-Qadi 'iyad in his a-Shifa added 5 other names which are more or less attributes among them القُثَمُ.