Raw catfish are a type of fish found only fishing spots within the Ruins of Camdozaal. In order to catch this fish, players require level 46 in Fishing and a big fishing net. Each catch grants 33 Fishing experience.
Unlike other fish, catfish are not eaten and are instead prepared at the preparation table for ritual offerings at the mysterious altar, requiring level 46 Cooking and granting 43 experience when successful. If the player is successful and offers the catfish and the mysterious altar, they get 16 Prayer experience. Players may ruin the catfish during preparation, resulting in a ruined catfish; the chance of failure while preparing these will decrease as players reach higher Cooking levels.
For an exhaustive list of all known sources for this item, see here