My question is, is the music custom made for the anime series, or does it exist outside the anime?
If it's an actual piece, what is its name/who is it by?
It looks like the OP of the Portuguese dub of Kaiketsu Zorro is the instrumental (i.e. no lyrics) version of the original Japanese OP.
The original Japanese OP was presumably made specifically for the series, seeing as it is titled "ZORRO" (cf. the ANN entry for Kaiketsu Zorro). The piece was sung by MASAAKI Endoh (probably better known for his involvement in JAM Project); composed and arranged by TOBISAWA Hiromoto, who also did the music for Kaiketsu Zorro; and featured lyrics by ARIMORI Satomi