The word "Houyhnhnms" refers to the intelligent, horse-like creatures that appear in Jonathan Swift's satirical novel, Gulliver's Travels. If you're looking for synonyms for this word, some options might include equines, steeds, nagas, stallions, or ponies. However, it's worth noting that the Houyhnhnms are a unique species and are not exactly the same as horses, so these synonyms may not be a perfect match. Nevertheless, these words can be useful for describing other horse-like creatures in literature or in real life.