Name's the Same: Superman had an evil counterpart also named Ultraman, who preceded the series by about two years.
Also, the main character of the TV series My Secret Identity called himself "Ultraman", but never actually used such an identity.
Prop Recycling: A few monster suits come from Godzilla movies, including the Big G himself at one point. Even the iconic V-TOL/Beetle is a modified prop from Calamity Star Gorath.
Apparently there were considerable difficulties in getting new monster costumes. Later costumes were, in a number of cases, simply recycled from earlier ones.
What Could Have Been: Ultraman was originally going to be in an Ultra Q episode, where he was going to be an alien villain, instead of a superhero.
Ultraman was originally going to be a bird like alien monster named Bemlar. Due to the fear that children wouldn't know who to cheer for, they turned Ultraman into a more humanoid alien. The name for the Bird man was given to Bemlar while it's design would be sold to Toho and be incorporated into the poular monster Rodan.