According to japanese legend what creature was used as the foundation of the earth?

It was the last couple, Izanagi and Izanami that gave earth its solid form and first procreated other gods.

The elder gods delegated the youngest couple Izanagi and Izanami to carry out their venerable mandate: to reach down from heaven and give solid form to the earth. This they did with the use of a precious stone-covered spear named Ame-no-nuboko (天沼矛?, "heavenly jewelled spear"), given to them by the elders.