On 'Bonanza' was Hoss Cartwright's real first name Hoss or was it something else?

It first name was Eric.

Dan Blocker ... Eric 'Hoss' Cartwright

https://www.imdb.com/title/tt0052451/?ref_=fn_al_tt_1

From site;
The origin of the nickname had two explanations on "Bonanza". "Haas" was a Swedish word for "big in stature and big in spirit". The word also became a southern rendition of "horse" and played on the character's girth.

Eric "Hoss" Cartwright was born in 1836, somewhere between California and Nevada. He is the son of Ben Cartwright and his second wife, Inger Borgstrom, a Swedish immigrant.

https://en.wikipedia.org/wiki/Hoss_Cartwright