This strange-looking vessel is usually described as a "fish with legs," but some sources refer to it as a "flea-like insect" instead.

The body dates back to the 10th century CE, but the legs were added a few centuries later, likely during the 14th century. The vessel itself was carved from rock crystal (i.e. quartz) while the head, legs, and frame are all made from gilded silver.

As this book describes:

Around 1400, a grotesque animal was created whose rock crystal body, with a simple head and three stilt-like legs, encloses relics of Mary Magdalene in a drilled cylinder.

Apparently, a rock crystal container dating to the 10th century — according to Gerard Lemmens, “an oriental perfume bottle in the shape of a fish," or, according to Henk van Os, specifically one of the coveted Fatimid flacons — was reworked. It appears that the fins of the flacon were ground down slightly on the sides to allow the two twisted metal cords of the holder to fit more closely along the back and belly.

This reliquary was originally commissioned for the nuns of St. Vitus Church, located in Emmerich am Rhein, Germany, where it was meant to serve as a display case for the alleged relics of Mary Magdalene. It was later moved to the nearby Church of St. Martin, which is where it's currently housed.

The vessel contains a piece of parchment that may be wrapped around a human bone (or bone fragment), but the exact nature of that relic remains unclear.

Sources & More Info:

De Gruyter: Dingkulturen (I have a digital copy of this book, so if you want any additional information or excerpts from the text, feel free to message me)

Wikimedia Commons: Fish Reliquary from St. Vitus in Emmerich am Rhein

Schnütgen Museum: Exhibitions: Magic Rock Crystal