This is the Art of Jizai Okimono- an art that emerged after the Samurai class was outlawed in the mid 1870's. The Myochin armorers made the battle clothing for the samurai, and when the class was abolished they needed to put their skills to other uses- this being the most artistic.
Most pieces are forged iron, but also bronze, copper & ivory- and most tend to be small insects & animals, sometimes very large Dragon figures.
Here's a lovely snake I was outbid on a few months back- it went for $17,500: Snake
Recently the Tokyo National Museum did a show on this particular art form- worth a look, as a collection like this exists nowhere: Collection.
If you're interested in this general area, but can't find (or afford) the Japanese originals- there are other options. In the mid 20th Century some makers in Spain started making some animals figures of very high quality in cast silver- I have a Crayfish available for sale on my website....and have recently sold a crab.