Raw guppies are a type of fish found only fishing spots within the Ruins of Camdozaal. In order to catch this fish, players require level 7 in Fishing and a small fishing net. Each catch grants 8 Fishing experience.
Unlike other fish, guppies are not eaten and are instead prepared at the preparation table for ritual offerings at the mysterious altar, requiring level 7 Cooking and granting 12 experience when successful. Players may ruin the guppy during preparation, resulting in a ruined guppy; 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