"Then I said, 'My lord, what are these?' So the angel who talked with me said to me, 'I will show you what they are.'
"And the man who stood among the myrtle trees answered and said, 'These are the ones whom the LORD has sent to walk to and fro throughout the earth.'
"So they answered the Angel of the LORD, who stood among the myrtle trees, and said, 'We have walked to and fro throughout the earth, and behold, all the earth is resting quietly.'
The rider of the red horse is almost undoubtedly Christ--the Angel of the LORD who stood among the myrtles. This would then imply that the horses are one of two groups: either angels or the church. Oddly, the amillenial commentators that I have read interpret the horses as angels (Meredith Kline, Glory in our Midst), while the postmills hold to the church (Jas. Jordan, Peter Leithart. Interesting tangent: Jordan also uses this section to interpret Revelation six (the four horsemen) as being Christ "riding" the church in each of the four sections).
There is very little in the vision to clarify which of the two groups the horses are, which actually is of no little importance, as symbolic Judaic literature remains fairly consistent, and borrows off of other symbolic Judaic literature--beasts are empires, seas are pagan nations, fish are unbelievers, and horses are--well, what are horses? Whatever they are here is most likely what they are in Revelation, as John is borrowing from this imagery--coincidences do not happen in Scripture. Why is the number six the number of man (and of the beast)? Because men and beasts were made on the sixth day. Goliath was six cubits and a span wearing scale armor and was killed by a head wound. Sisera was killed by a woman's tent peg through the skull. There was a scarlet cord tied around the younger of two twins who ends up in the line of Christ, and what do we find in Rahab's window--the window of another ancestor of Christ? No, we're not going for subtlety here--if there are two similar sections of Scripture, they are linked and should interpret each other.
The colors of the different groups are very interesting (at least if you're reading Kline--otherwise they're just kinda colors). There is white, red, and "sorrel" or "bay." Kline notes that these colors may be used to imply flame and light: "A desire to produce such a bright, fiery image would then explain why black horses, which are found in the seventh vision, are absent from this first one. Also, the second color term"--the one rendered "bay" or "sorrel"--"seruqqim, evidently derives from a verbal root that is used for the shining of the sun."
If Kline's take upon this is correct, then the horses are a manifestation of the Glory of God, of His Presence. This does not definitively demonstrate the horses to be angels or men. However, what would we normally say is the manifestation of God's Presence and Glory on earth, and do we have any reason to shy from that in this instance?