Warhound Scout Titan
Ever heard of reconnaissance-in-force? Well, this is the Imperial cranked up to eleven version of it. The Warhound class Scout Titan is the smallest Imperial Titan in use (although it still reaches around fifteen metres (fifty feet) at its peak) and the fastest of them. While they're unable to take and dish out the insane amount of punishment one of their bigger brothers like a Warlord can, its speed and maneuverability allow the Warhound to perform devastating hit-and-run and flanking attacks, along with performing recon and acting as the eyes and ears for the bigger behemoths. During the Horus Heresy, Warhounds tended to operate (semi-)independently from the bigger Titan Legion in large packs but those days they work in tandem, the pair often taking a complementary weapon loadout and putting the fear of the Omnissiah into everything smaller than a Titan bigger than themselves by murderrapefucking the hell out of them.
The 'Scout' designation might be taken as a joke, considering this is a 50 ft. tall machine and any enemy that can't see or even HEAR this thing 'sneaking' around (Here's looking at you, Creed) deserves what comes out of their fuckheug rape cannons. True. However a scout's role is to gather info and perform reconnaissance, and not necessarily sneakily. This is a job for which the Warhound is the most suited for amongst the different types of gigantic walking engines of mass destruction in the Imperium's arsenal. (And yes, they could use Skitarii or allied Imperial forces to do it. But Titan Legions are independent like that...)
A Warhound is captained by a Princeps, crewed by two Moderati, maintained by two Techpriests, and operated by three Servitors. A unique feature is its digitigrade leg configuration, which affords the Warhound a spectacular speed for a walker of such size; it even has the ability to transfer all power from the plasma reactor into the legs, allowing it to bound across the battlefield with ease. Unlike the other Titans, which feature human shapes, the Warhound more closely resembles a jackal, with a head poking out from under hunched shoulders.
One notable variant was used by the Legio Audax (the Ember Wolves) who fought directly for the World Eaters. Up-Armoured and equipped with a Vulcan-Mega Bolter and a custom weapon called an Ursus Claw - a cross between a whaling harpoon and a railgun - they were deployed in large groups to overwhelm and flank their prey. The wolf theme was carried on with the design of their "heads" as well as their battle tactics - a small group were capable of bringing down an Imperator Titan to its knees, rendering it unable to fight and easy to capture.
Overview
The Warhound Titan can only be used in Apocalypse and Escalation, and is one of the deadliest things on two legs, having nine hull points and two void shields. While all its weapons are strong, just stick with two Turbo-Laser Destructors for their Destroyer rule. While it doesn't have as large a blast radius as the full-fire Plasma Blastgun, it's still a big radius, and the Destroyer rule means you stick rolling to wound and rolling for penetration on ANYTHING the template TOUCHES. This thing will take fire and probably not survive the game, but expect it to take a chunk of the enemy army with it.
Can also be hidden, although that would require a tactical gen- CREEEEEEEEEEED!!!!!
The Warhound Titan has two hardpoints on its arms for weapons, the possible weapons that can be mounted are the Vulcan Mega-Bolter, Plasma Blastgun, Inferno Gun and Turbo-Laser Destructor.
Chaos Warhound Titan
Also known as the Feral-Class Titan, the Warhound Titans in service to the Ruinous Powers tend to be used in much larger numbers to strike fear into its enemies. One of the reasons why today's Mechanicus broke up the Titan Legions like the Astartes broke into Chapters is because Horus let traitor Warhounds scare the shit out of Terran civvies in groups of ten. Chaos is kind of a dick like that.
With the arrival of Imperial Armour Volume 13, you can now swear a Chaos Warhound to the service of a chaos god. This gives them a god-specific benefit and grants them the Demon special rule.
The Machine Spirit of a Feral Titan is wholly corrupted by the powers of Chaos and the Titan's soul and crew, now possessed by daemons, are doomed to serve the Chaos Gods in battle for all eternity.
They are formidably armed with two 'Titan weapon' mount hard-points available, with the same weapons available as the Imperial version.
As with the Imperial Warhound, they excel in urban environments. Their relative speed and agility enables them to fulfill a similar scouting role to their imperial counterpart, but as is the nature of chaos, they also have a tendency to act like literal Jackals, ruthlessly picking off weaker targets.
As the Warhound advances it growls and roars (Seriously who let Goto write this? Granted we aren't sure whether this is a win or a fail), broadcasting a dirge of Chaos litanies and prayers, driving the worshippers of Chaos into a bloodletting frenzy, for surely nothing can stand before them when a Titan bestrides the battlefield.
Essentially they are like the guard dogs of Chaos without a leash.