War Boar

The War Boar is a distinctive breed of the more common Wild Boar that has grown to such a size that they are as large as an Imperial warhorse, but are far more vicious in their behavior, and far more bulky in their physique. These especially large and aggressive boars can be found almost exclusively in the harsh rugged regions of the Badlands, where they are often trained by Orcs to be ridden as combat mounts. In large battles involving Greenskins, units of specialist War Boar-mounted Orc cavalry known as Boar Boyz can often be seen charging into the fray as heavy shock cavalry.

Most Orc settlements have secure pens where they keep their boars in when not used in combat. The boars themselves are evil-minded creatures that take every opportunity to maim, bite and kick their masters, but this doesn't really concern the Orcs who are on the whole sensible enough to keep well clear, preferring to leave the job of feeding and caring for these creatures to Goblins and Snotlings. Omnivores to the extreme, these beast can eat almost anything they can get their mouths on.

Source

 * Warhammer Armies: Orcs and Goblins (8th Edition) -- pg. 37