Different tail - Ibi

Japanese: 異尾 - いび

…The tails of Paleozoic horseshoefish (Osteochondria) are asymmetrical, with either the upper or lower lobe being larger than the other, and the spine is bent upwards or downwards at the base of the tail to support the larger lobe. A tail with a large upper lobe is called an heterocercal (irregular tail), and a tail with a large lower lobe is called an inverted heterocercal (inverted irregular tail). Sharks have heterocercal tails, but no fish with an inverted heterocercal tail are alive today. …

From [Acanthodid fish]

…The scales are tiny, but have a hard enamel-like layer on the surface, with a bony base underneath. The tail fin has a large upper lobe and is a heterocercal tail. This heterocercal tail is a primitive form of tail fin, and is also seen in primitive groups of cartilaginous and bony fishes. …

*Some of the terminology explanations that mention "Io" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…古生代のカブトウオ(甲皮類)の尾は上下非対称形で上葉または下葉が他の葉より大きく，脊柱は尾の付け根で上または下へ折れ曲がり大きいほうの葉の支柱になっていた。このように上葉が大きい尾を異尾(不正尾)，下葉の大きい尾を逆異尾(逆不正尾)と呼ぶ。サメ類は異尾をもつが，現在，逆異尾をもつ魚類は生存していない。…