A speed horse is a fast horse, and a fine horse. This word isn't very common, but it does come up in horse racing contexts, and creative writing.

This is a jukugo word that uses the on'yomi readings of the kanji, which you already know. However, 馬 uses a bit of a special reading here: め. So here’s a mnemonic to help you remember this:

This fast horse is so fast that all you can see whenever it speeds past you are its shiny bright 目 (め). Imagine this fast horse zooming past you over and over, but all you ever see are its white 目.