A fictional artist is a person (or group) that does not exist in the real world but is in one way or the other of discographic relevance - e.g. characters from a movie - and therefore exists as a valid artist in the database. The actor, voice actor, or creator that exists in real life isn't directly credited to the releases or track but is linked to the fictional artist through a Voice Actor Relationship Type.
This could lead to a chain of characters such as with the members of Spın̈al Tap: Christopher Guest performs the voice of Nigel Tufnel who is a member of Spın̈al Tap. As stated in Don't Make Relationship Clusters, and to avoid potential problems, make sure the chain is linear and doesn't take shortcuts: for example, Christopher Guest is not a member of Spın̈al Tap.