Hunter

The Scarab is meant to be smaller, it's not the same Scarab. This scarab is also one of the ones that have humanoid pilots, as opposed to Lekgolo worms (the little worms that make up Hunters) which pilot the Halo 3 ones.

The Scarabs you see in Halo Wars are repurposed mining equipment. Only in Halo 3 do you see a fully militarised Covenant in terms of build purpose for their equipment.

If you do a bit of wikipedia research on the phantom and the scarab you'll see what I mean.