{ Assuming you have created TMyCreatureDescendant with MyCreatureDescendantResource, see http://castle-engine.sourceforge.net/tutorial_resources_extending.php for explanation }

function TMyCreatureDescendant.Enemy: T3DAlive;
begin
  Result := inherited;
  { allies attack player enemies, or nothing }
  if MyCreatureDescendantResource.Ally then
  begin
    if World.Player <> nil then
      Result := World.Player.LastTarget else
      Result := nil;
  end;
end;