The Heisenberg family crest is a iron horse head surrounded by an iron horseshoe. The epithet "Iron Steed" refers to Heisenberg in some of the in-game books.

(IIRC the Japanese version uses Hagane no Uma, "Steel Horse". Most metaphorical allusions to "steel" in Japanese tend to be translated as "iron" in English because the metaphor works better that way. Like the "Iron Will" skill in Umamusume.)