Rosario is a giant red fish with a hard metallic shell. The shell has jewels embedded on the top part and on the forehead.
Little is known about Rosario’s personality, but she seems intelligent enough to follow Muchigoro’s orders.
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.
Baron Omatsuri and the Secret Island
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. The crew tried multiple times to catch Rosario but failed to do so. When Luffy tried to use the a small net that was given to them earlier, he ripped it. Muchigoro lured his pet into his much larger 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 when Chopper used his Guard Point to bounce the fish into their bucket.
"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.