Carp
鯉 koi

From the legend of the carp that swam up a waterfall and became a dragon, the koi stands for perseverance and success, especially wishes for sons on Children's Day.
Chinese Dragon Gate legend (Han dynasty); Japanese carp streamers Edo period
The carp's meaning comes from the Chinese story of a carp that swam up the falls at the Dragon Gate on the Yellow River and became a dragon. In Edo-period Japan samurai families flew banners for Tango no sekku (5th day of the 5th month), and in Edo these merged into carp streamers (koinobori) for sons. Colourful carp streamers spread to ordinary households in the Meiji era.
Flown for Children's Day (5 May, a national holiday since 1948), now often one carp per child; carp are a spring and early-summer motif.