Zillions-of-Games values the Cannon more than the Nightrider. The main advantage of the Cannon is that it can attack other pieces without being attacked back by them. The Nightrider can do that from a distance, but up close, many pieces it attacks could capture it.
I seriously doubt that not being able to attack from close distance (which is peculiar to this variant, where almost every piece has Knight moves) is a larger handicap than not being able to attack without a mount. The BN, RN and Equus Rex are worth more than a Nightrider anyway, so you could still attack those from close by when protected. And Nightriders have enormous forking power, from close by as well as from a distance, (and can also skewer pieces), so the Knight compounds (plus Queen) are very vulnerable targets for most of the middle game. A Cannon has very poor forking power, with only a single forward ride instead of four.
That an occasional mate position exists, which cannot be forced, has no impact on piece value at all. Even mating potential (i.e. the ability to force checkmate against a bare royal from almost any position) in general contributes very little to piece value. Because there usually remain enough promotable pieces around to provide such mating potential, and the main value-determining trait is how well the piece can support those on the way to promotion.
Without using zugzwang an Equus Rex cannot be driven to the edge so quickly, so I doubt that even Equus Rex + Cavalier can be beaten by Equus Rex + Cannon from most positions. The Cavalier would usually promote to Queen in time to launch a counter attack (and win easily). A Cannon alone would be powerless to stop such promotion. So statistically the advantage might even be with the player that has the Cavalier. With a piece stronger than Cavalier it would be very easy to prevent being 'cornered'; you would just aim the piece at the square where the attacking Equus Rex could take opposition. If it is not Cannon vs Cannon (which by symmetry should in general be a draw), the Cannon would probably be massacred very easily.
I seriously doubt that not being able to attack from close distance (which is peculiar to this variant, where almost every piece has Knight moves) is a larger handicap than not being able to attack without a mount. The BN, RN and Equus Rex are worth more than a Nightrider anyway, so you could still attack those from close by when protected. And Nightriders have enormous forking power, from close by as well as from a distance, (and can also skewer pieces), so the Knight compounds (plus Queen) are very vulnerable targets for most of the middle game. A Cannon has very poor forking power, with only a single forward ride instead of four.
That an occasional mate position exists, which cannot be forced, has no impact on piece value at all. Even mating potential (i.e. the ability to force checkmate against a bare royal from almost any position) in general contributes very little to piece value. Because there usually remain enough promotable pieces around to provide such mating potential, and the main value-determining trait is how well the piece can support those on the way to promotion.
Without using zugzwang an Equus Rex cannot be driven to the edge so quickly, so I doubt that even Equus Rex + Cavalier can be beaten by Equus Rex + Cannon from most positions. The Cavalier would usually promote to Queen in time to launch a counter attack (and win easily). A Cannon alone would be powerless to stop such promotion. So statistically the advantage might even be with the player that has the Cavalier. With a piece stronger than Cavalier it would be very easy to prevent being 'cornered'; you would just aim the piece at the square where the attacking Equus Rex could take opposition. If it is not Cannon vs Cannon (which by symmetry should in general be a draw), the Cannon would probably be massacred very easily.