A Japanese fish-shaped cake. The most common filling is a red bean paste made from sweetened azuki beans.
These wagashi (Japanese sweets) are a favorite of Tsukimiya Ayu from Kanon, Konjiki no Yami from To LOVE-Ru, Sakura Kyoko from Mahou Shoujo Madoka Magica and Mikami Ryou from Kamen Rider Blade (to the extent that he becomes Taiyaki Meijin).
This tag implicates wagashi (learn more).