When people find a shachihoko on the roof, they sometimes cofuse shachi and shachihoko.
But shachi is a real animal and shachihoko is an imaged animal.
It is a influential belief that the original image of shachihoko was made by the image of Makara(1) in India
Its head is like a dragon and body is like a large fish in the sea.
It is said that it's their natural behavior that as soon as they find fire they spout water.
So, on the top of the roofs of castles or temples,
they were settled as charm of protection from fire.
Gold shachihoko are a symbol of Nagoya.
You can see a pair of shachihoko on the top of the roof of Nagoya castle.
They were put there mainly for the purpose to show the power of the lord of the castle,
and also for a kind of decoration
They also took a role of war funds in an emergency
Nobunaga was the first lord who put shachihoko on the donjon roof (Azuchi castle).
Osaka castle, Fushimi castle and Edo castle also had shachihoko on the roof,
but they were burned out by fire.
In the middle of the Edo period, Nagoya castle became the only castle
which has golden shachihoko on the top of the donjon roof.
The real golden shachihoko were melted out by the fire during the World War II
and rebuilt in 1959.
(1): Makara is often translated water horse, and is also an apotheosisized imaged animal,
partly elephant, crocodile, hippopotamus, manatee, etc.