Mage's Chariot

A caster's chariot is larger than most typical chariot designs and is typically pulled by three or four horses, though one- and two-horse designs also exist.
Though chariots are generally considered an obsolete form of vehicle technology, long since supplanted by wagons, saddles with stirrups, and carriages, the unique construction of the chariot provides a few key advantages for spellcasters on the battlefield.
The biggest of these advantages is that of a section of "ground" that moves. This can be very effective for area-of-effect spells, though a mage must be careful about getting the horses caught in the spell radius in some cases. To counteract this problem, some designs have an adjustable draught-pole that can be extended to get the horses out of range. Other work-arounds include spells that can exclude certain creatures from their area of effects, magic-resistant barding on the horses, and specially-trained casters.
The second major advantage is that the caster can take security with them. Typically this takes the form of a heavily-armored soldier with a large, heavy shield and a crossbow. There are a number of shield designs that integrate the crossbow and an archer's slit into the shield itself and others that simply provide a built-in rest. The soldier is also typically armed with traditional hand weapons to provide security in the case of the chariot being brought down.
Similarly, the horses are in a relatively quick-release harness and they are already saddled when they are hooked up to the chariot.
A third major advantage is that the flat platform of a chariot makes integrated holders for various spellcasting aids such as component boxes, wands, scrolls, and the very easy and convenient for the spellcaster on board. This same phenomenon also makes the chariots tempting targets on the battlefield, however.