Naruto: Five psychic beasts that cannot be summoned

1. The Great Toad Sage: The leader of Mount Myoboku, one of the three great sages in the mortal world. Jiraiya, Tsunade, Gamakichi, and Gamatatsu are its descendants.

 2. Gama Bunta: A giant toad that can be summoned, but only a small part due to its size. Its true form has never been seen by anyone except toads.

 3. Enma: The Third Hokage's exclusive summon. No one could summon Enma after his death. The lineage of monkey summons may have been lost. 

4. The Slug Sage: The children of the Slug Sage have formed contracts with the Fifth Hokage. Its size may be the largest among summons, but nobody can summon its true form.

5. The White Snake Sage: It appeared but no one has formed a contract with its true form. Orochimaru discovered its sacred place but couldn't summon it. It is one of the most mysterious sages.