well, to me, At least Ryuki has the bug-eyes and rides a bike (though his design is a dragon) and 555, he has feelers (revelation that he's a shark instead of a firefly) and still looks like a bug. As for Momotaros, Although his design is a peach, but the peach split into 2, with feelers extending out, makes him look like a praying mantis. Well, Kiva... Big eyes but most importantly, he rides a chopper. Amazon, although a chameleon, his still had bug-like eyes and a bike.
But I really cannot convince myself to call Hibiki a Masked Rider. Don't get me wrong. I like the show, but I still don't think he fits in the category... Granted, he's masked, but then he doesn't resemble a bug, nor ride a bike (well, he learnt to ride a bike later due to rumors of viewers complaining he didn't).
As for Apollo Geist, He's a villain. Masked Riders are supposed to be the good guys. If he can be considered a Masked Rider, then the Shocker Troopers can be Masked Riders too, cos they wear masks and ride bikes to. And that'll defeat the purpose of delivering the message of Masked Riders being the allies of justice.
Of course, arguing about Kaixa, TheBee etc that they aren't exactly good guys, but then the concept is that they are rogue Riders. They could be the good guys if they wanted, but they stray from their paths.
Shadowmoon is a different thing altogether. He's a bug, he owns Battlehopper, too, in a sense, but he was born evil.
Maybe you all can say that I'm too fixated about the overall image as being 'insectisized', but look at it this way. Would you still call an Ultra, an Ultra if you remove the salted egg-like eyes, the color timer and the dorsal-fin-like thingee on their heads?