Here's the reason why I assume it's a stallion and also the reason why there's no character card. I could only get Agnix in a set with this car and trailer.

Oh, I just saw that for some other Fillies their relationship is marked as "sisters". This isn't the case for Agnix, so now I'm pretty sure that he's the husband of Bianca and father of Loki and Samba. Well, then he can drive those two hurricanes from party to party in his posh car. ;D