Both the Shadow Fox and the Mirage Fox are the Helic Republic's (or someone else's) attempt into reverse-engineering and recreating their own version of the Guylos Empire's Zeekdober
How you may ask? Simple. Just look
at
them.
Notice some of the similarity between the Zeekdober and the Shadow Fox? And with the Mirage Fox even more so? The fact that the 3 of the Zoids were use for as stealth-purpose high-speed high-performance Zoids that can equipped with wide array of weapons make this connection even stronger.