Froggy Rock is a summoning creature that Jiraiya has called upon in Naruto. However, what was summoned was only a part of its stomach because of its immense power. Froggy Rock is so powerful that its stomach alone almost trapped both Kisame and Yagura, two prominent characters in the series. The true form of Froggy Rock has never been seen or summoned. 

Enma, the Monkey King, was the summoning creature of the Third Hokage.

Not only was Enma incredibly powerful on its own, but it could also transform into a weapon for battle. After the Third Hokage's death, no one else has been qualified to summon Enma, and there won't be anyone in the future. It is a great loss. 

Slug Sage, one of the Three Sages, is just a part of Katsuya, Tsunade's summoning creature. Katsuya itself is powerful, possessing the abilities of a sage.

To sign a contract with Slug Sage, one must not only master sage techniques but also possess enough power to match them. Currently, no one is capable of summoning it. 

Toad Sage is the oldest toad in the Toad Group. It once had a contract with the Six Paths Sage, but its age has rendered it almost powerless. As a result, no one can form a contract with it, not even Yamashiro, who is of the same generation.