If someone rigged one...

wouldn't it be a quadruped? Yet it would still be a .biped...

Hax.

Half horse half man is a Centaur. Quad= four. It would be a quadruped.

..... so what exactly are you asking? how to make a .biped out of something with 4 legs? its called a .biped because it only has support for 2 legs. the other two can be rigged to separate bones and animated separately of course, but the game will still treat it like a 2 legged beast.
where did the quadruped vs biped thing come from?

You'd have to "fake" the animations of the four-legged thing to where CE only recognizes it as a biped. Like make the animations of it's movement seem like it's moving two legs when in CE's account it is moving actually one. It's hard to explain without having an example to go by. Each pair of legs is animated to where when one moves forward the other moves in a corresponding motion, so when you go to CE it recognizes it as a biped. Of course, this is probably overcomplicated and not even needed to be put to this measure. I don't know that much about the creation and exporting of bipeds, so please excuse me if this seems far fetched.

and infection form

A sentinel floats so it does not require the same as an Elite biped for instance. Infection forms run on a traversed plane (?). I could easily be wrong on both of these.

Am I missing something or does HaloCE only recognize 'Bipeds' (not the other bones)?

see this is why its a good idea to avoid stupid questions like "imma make a biped that has 4 legs, isnt it technically a quadruped? why is it still called a .biped?"
these questions basically confuse the hell out of everyone that doesnt know alot about bipeds.
flood infection forms have two planes as legs with an alpha channel that makes it look like they have 6 tentacles

aye what donut said, but more than anything: it's just a name for the specified set of tags. ITS NOT IMPORTANT

Yeah, it was just a random thought.

but seriously, i can tell at least darkhalo was confused, and im sure quite a few others were too. thats basically the point im making

What about the turret guns on the spirits in campaign, there bipeds. there not bi or a ped

They used bipeds so they could make it an AI.