Slavegiant

Giants are enormous, lumbering brutes that stride the world seeking battle, food and strong liquors. They are humanoid in appearance, although their features are coarse and misshapen. They are dull-witted even when compared to an average Ogre, which is really saying something. An Ogre Tyrant, if he can restrain himself from attacking the Giant to prove his own superiority, will often try to recruit the solitary wanderers into his tribe. Sharing food or intoxicating drink with the enormous lummox often does the trick, as do promises of an easy life full of eating and drinking. All that is required of the Giant is that he helps the tribe in battle. More brazen Tyrants might actually intimidate a Giant into joining the tribe, either by displaying his own fierce fighting prowess to physically best the brute, or by scaring the behemoth with exaggerated tales of his tribe's past experience eating Sky-titans.

Giants that do join an Ogre tribe are often branded with the tribe's symbol. This is done partly to instill some tribal pride in the non-Ogre, but perhaps more so to remind his own tribe-mates that he is a friend and not a foe. In the heat of battle, such things often escape an Ogre's attention and there is a natural compulsion to fight (and eat) a Giant. After all, it is every Ogre's dream to boast of felling 'a big one'.

Source

 * Warhammer Armies: Ogre Kingdoms (8th Edition) -- pg. 46