Abarim Publications' online Biblical Hebrew Dictionary
Linguists identify two separate words סוס (sus) but neither BDB Theological Dictionary nor HAW Theological Wordbook of the Old Testament discuss or even identify their root(s). The valiant Alfred Jones (Dictionary of Old Testament Proper Names), however, quotes the famous theologian Gesenius, who equated at least one root סוס (sus) with an Arabic verb meaning to leap onward. Here at Abarim Publications we're pretty sure that there aren't two words but only one:
The masculine noun סוס (sus), denotes a bird: a swallow or swift. This word occurs only in Isaiah 38:14 and Jeremiah 8:7.
The identical masculine noun סוס (sus), is the Bible's common word for horse. Its proper plural is סוסים and its pseudo-genitive is סוסי (horses of).
Our noun shows up all over the Semitic language spectrum, and was obviously imported into Hebrew. The feminine equivalent, denoting a mare, is סוסה (susa) and occurs only in Song of Solomon 1:9. A less common word for horse is פרש (parash), which denotes the horse(man) as unit of military force, or army-quantum.
Horses were known to the Hebrews since their stay in Egypt (Genesis 47:17) but weren't incorporated in Israel until the time of the kings (2 Samuel 15:1). Horses became symbols for a nation's military force, and some of the Bible's most perplexing visions feature horses. Read our unique discussion of the four horsemen of Revelation 6.
Since the Hebrew language doesn't name things after their appearance (as modern Western languages do, and that's how we got the sea-horse and the horse nebula), but after their behavior, any animal that zips about fast and vigorously, may be known as סוס (sus): horses, swallows, and who knows what else. See for a more extensive discussion of this phenomenon our article on To Be Is To Do.