The C38 beastman quadruped is quite unique in bodyplan so I can't think of a direct analogue that I know of. Like a guy walking on all fours. But if I interpret it more freely I can think of a couple of things:
A centaur is technically also a quadruped. It's also already something "of chaos" so it's not a big stretch to imagine it would lead a chaos warband. And, the C38 beastman looks a bit challenged as the hero sort, what with not having any hands to grasp weapons with and such. The centaur comes across a tad more believable as something that would kick some actual butt.
I can think of the metal citadel centaurs, boar centaurs, 4th ed bull centaurs and later plastic centigors but neither of them tend to come cheap so there's that too. But there is plenty choice out there amongst other brands if you are not averse to straying from citadel models. For example, Mantic does plastic-resin centaur thingies. "Abyssal halfbreeds" they're called, I think. And they have a metal hero option too.
Off-the-shelf centauroids would be my first tip.
If you're open to converting something that could also be a way to go. Perhaps that's not an option for you but in case... these would be my thoughts. For example, I made some centaurs myself from bits box junk parts. Requires work, but dirt cheap if you can use stuff you already own.
And hey, it's chaos so it's not like you're hemmed in. A plastic wolf or boar would also make a good conversion base for a centauroid quadruped type guy.
For a different strain of thought I was thinking that a plastic cold one could make another good base for a conversion for a weird and wonderful chaos champion. In actuality it's more of a bipedal locomotion creature but the front legs look like they could be used for support to bend down and touch the ground. In my book it would be passable for a quadruped chaos mutation. Close enough, in that it's pretty far removed from an erect bipedal human bodyplan. I would snip off the front lizard claws and splice on weapon-hands. And the neck could either transition into a torso, or just a humanoid head.
Something like this... obviously, we're deep in conversion territory with this guy. If not... plastic barnyard animals!