The fish-themed McDonaldland character who opposed the Hamburglar was Captain Crook.
Captain Crook was a pirate-styled character associated with Filet-O-Fish sandwiches in McDonald's advertising. His role connected a fictional personality to a specific menu item, a technique McDonald's used repeatedly in its McDonaldland campaigns.
The Hamburglar was a separate character whose signature behavior involved stealing hamburgers. Captain Crook was therefore not the burger thief, although both characters appeared in the same advertising universe and were sometimes shown as rivals.
McDonald's introduced McDonaldland characters during the 1970s. Over time, the company reduced the visibility of many of them, while Ronald McDonald remained the best-known face of the brand.