From Halopedia, the Halo wiki
The Chimera was a Covenant unit cut from Halo Wars. It was a four-legged walker bearing slight resemblance to the Type-47A Scarab, though with a more hunched appearance.
The Chimera (bottom-left), seen alongside a handful of other WIP renders of various Covenant vehicles.
An icon for the Chimera from the in-game unit selection.