Rhenium is the second last fully stable elements on the periodic table. It is named after the Rhine where it was first discovered. It is used in many alloys as well in catalytic converters used in cars. It’s costly due to the difficulties in refining, and it’s the rarity. Its rarity probably could even sometimes make some consider it Rhinegold, which features in some Norse folktales.
Haddock are sizeable cod-like fish that have become a staple food source in some countries. They are tied to Rhenium as they have been known to swim into the brackish waters near the Rhine entrance to feed on the small freshwater fish who live their often in abundance.
Osmium is a hard silvery metal that is mostly used in industrial manufacturing, fountain pen nibs and electrical contracts. Although I was tempted to tie Osmium with a mammal or reptile, Ostriches seemed a better match as they land-bound birds, often considered very hardy and their feathers were previously used in quills before the popularity of quills ceasing. Ostriches are also often depicted putting their heads in the sand, which is strange as they are incapable of doing this.