At some point, Rosario joined the Red Arrows Pirates and became Muchigoro's pet. This was likely after the crew was revived, since the goldfish doesn't have a plant on his head like the rest of the revived crew.
When the Straw Hat Pirates participated in the first Trial of Hell issued by the Baron Omatsuri, they were initially tricked into thinking they had to catch small goldfish, but, instead, they were tasked with catching Rosario against Muchigoro. Muchigoro lured his pet into his net and tried to throw her into his bucket but Nico Robin blocked it. After some tossing around of Rosario, the Straw Hats eventually caught her due to Chopper's Guard Point.
"Rosário" is Spanish, Portuguese and Italian word for "rosary".
Despite all the revived Red Arrows Pirates having sprouts on their heads, Rosario does not. Since Baron Omasturi states that all his crew was killed before he used Lily to revive them, Rosario was probably not used by Muchigoro when he was alive.