Dave - rendjur is a construction I have never heard in the 40+ years I have lived in Sweden, and you will not find it in regular Swedish-English on-line dictioniries. Nor is it recognized by major Swedish dictionaries including Svenska akademiens ordlista, the official dictionary issued by the Swedish academy. Reindeer in Swedish is simply ren. True, Swedes construct words by concatenation, but the nearest you get (except possibly in dialect forms) to rendjur is rentjur, meaning a male reindeer (tjur means bull).
You also make a rather tenuous leap from “uncertain origin, but is likely a reference to the creature’s antlers” to “a reindeer is literally a ‘horned beast.’” Not up to your usual high standards of critical analysis!