* As a fish: "The catfish swam upstream." (Not capitalized)
* As a person: "I think I'm talking to a Catfish online." (Capitalized as it's a specific, proper noun)
* As a verb: "He was catfished by someone online." (Not capitalized)
In general, "catfish" is not capitalized unless it is used as a specific name or as part of a proper noun like "Catfish" the band.