Several units in the Orcs & Goblins army have the option to be upgraded to Big 'Uns. You may upgrade a single unit of these (not one each) in your army to Big 'Uns.
Source: Warhammer Armies Project: Unofficial 9th Edition (v2.2)
Several units in the Orcs & Goblins army have the option to be upgraded to Big 'Uns. You may upgrade a single unit of these (not one each) in your army to Big 'Uns.