There are 18 elemental types in the Pokemon games, which means a total of 171 unique combinations are possible. However, even with over 800 Pokemon not all type combinations have yet been used. This page lists all the unused type combos in the game. Its fun to think what Pokemon with these types would look like!
We also list some combinations that have been used, but only for legendary Pokemon or Mega Evolutions. Note that where a type appears in both sections it means there is a legendary and a Mega which is that type (e.g. Zekrom & Ampharos), but no regular Pokemon.