Yes, there were plenty of Mastodon's then, and they inspired the Central Pacific to make theirs. What Sam probably means there was only one 4-8-0 on the CP. The engine was successful, but they wouldn't let its builder make any more, instead had him make the biggest engine of the time!
I dunno if I'd consider that 4-10-0 a faliure, but it definitely was ahead of its time! Its boiler was too small, as was its tender, which really limited its power, etc.