Corbin was pregnant with Marshal.
Part of Corbin's shapeshifting abilities allowed him to change sex. At some point, while Corbin was in a female shift, he became pregnant, and the result was Marshal. He's referred to as Marshal's father because he prefers male pronouns and doesn't in any way consider himself Marshal's "mother." John impregnated Corbin.
To start, see the above WMG regarding Corbin's shapeshifting abilities. Marshal is described as being built like the Rook, but the book's pretty clear that Corbin's body type and the Rook's body type are not the same. We know that Ernest is taking after his father in build, which would lead him to being a giant of a man like John and like Marshal. Word of God has it that Amira/Corbin/John is as close to being a canon OT3 as it can be without being canon, so obviously there's interest on both Corbin and John's parts.