There are two options: John himself, or the horses that appear in Revelation 6:2, Revelation 6:4, Revelation 6:5, and Revelation 6:8. Given that John is already present and watching, and that a horse appears directly after the call is issued, it is logical to understand that the living creature summons the horse. The implication is that each horse is pictured as already backstage,
waiting to be summoned.
1 Now I watched when the Lamb opened one of the seven seals, and I heard one of the four living creatures say with a voice like thunder, “Come!”