The Hellhound is a monstrous creature from mythology, taking the form of a massive wolf or dog with connections to demons or devils (assuming there's a difference). It frequently appears in various tabletop games as a relatively low-tier extraplanar monster, often with multiple heads and/or the ability to vomit fire over anybody it doesn't like. In Dungeons & Dragons, they're one of the oldest and most recognizable "infernal beasts", far more than their counterparts the Hellcats, and noted for being extremely evil but pack-orientated, and also for spewing fire everywhere. In Pathfinder, you have both the fire-breathing hellhounds and the Cerberus, a bigger, nastier variant with multiple heads, no fire breath and no skin.
It's also a badass name for a vehicle or mecha, and so frequently gets used for that purpose.
In the universe of Warhammer 40,000, the Hellhound pattern tank and its variations are a series of light tanks fielded by the Imperial Guard; its main purposes are to provide fast fire support for the Imperial Guard, and to fuck the shit of anything that's hiding in cover.
There are several different patterns of the Hellhound in use by the Imperium, such as the Artemia Pattern which replaces the vehicle's turret with one that is remotely controlled from inside the vehicle by the vehicle's commander which allows the tank to make do with a crew of only two. Other patterns of the Hellhound, such as the more common Mars Pattern, use several smaller fuel tanks that can be hidden inside the remnants of the Chimera chassis' troop compartment. This pattern of Hellhound is preferred by Imperial commanders to the standard pattern of the vehicle since it is similar in appearance to the regular Chimera transport, making it harder for the enemy to identify.
The Hellhound uses a modified Chimera chassis: replacing its transport capabilities with a heavier engine to allow faster movement, promethium tanks (or equivalent, depending on the weapon in question), and heavier side armor. This increases the survivability of the tank for its primary purpose: to get close to the enemy and melt their faces off. The crew that volunteers for Hellhound duty tend to be a rather unhinged bunch that love to set people on fire and watch them burn to death. There are currently three patterns of the Hellhound in use: the Hellhound, the Bane Wolf and the Devil Dog. Like the Chimera, the Hellhound and its variants can field a hull-mounted Heavy Flamer, Heavy Bolter or Multi-Melta.
BRO TIP: The only way these three tanks differ from each other is by the barrel of their weapons. Make them in such a way so you can swap between the flamer, chem and melta barrels to get more mileage out of your vehicles. The head of the weapon is a simple plug-in so it should be easy to just not glue them in place and swap between them as needed.
The Hellhound is the classic pattern: by using a unique flamethrower that allows the vehicle to project its burning payload over a very wide area, allowing the introduction to potentially over a dozen enemies to your good friend Mister Yellow. This will fuck up anything that's not an MEQ, including Tyranid and Necron Warriors, most Aspect Warriors, any Tau infantry, and any blob of the non-MEQ or moe variety. By far the most popular choice for pick up gamers.
Hellhounds have gotten a whole lot cheaper on the whole and faster as well. That's good, because they'll need it. The once-frail Hellhound has suffered under the change to tanks as well, meaning that massed infantry fire can pose a serious threat to even its T7 W11 3+ frame. Out of the box a Hellhound costs a mere 101 points, which really helps for what it does. It can pick one of three secondary weapons: a Heavy Bolter, Heavy Flamer or Multi-Melta.
The Hellhound's Inferno Cannon packs a punch at Heavy 2D6 S6 AP-1 D1, making it effective against most infantry. Sheer number of shots will burn through hordes and allow you to ping medium and heavy infantry well enough. And with 16" range it doesn't have to come too close either. The first pick for a secondary weapon would be the Heavy Flamer with a similar profile, but do note that it's got only half-range compared to the Inferno Cannon. It's also more expensive than the standard Heavy Bolter, nudging your Hellhoud to a total of 110 points. The benefit for all of this is that whether it has 11 wounds remaining or 1, this setup is unaffected in terms of damage output.
When you want to fuck up MEQ. The Bane Wolf Chem Cannon is very likely to wound. Can be surprisingly effective against nearly any MC your Tyranid opponent can and will throw at you, on the accounts that it *will* wound. But do not forget, it can only cause *1* wound, so make sure to only do this if your target is down to its last. Unless you're feeling particularly ballsy and squadron them. Beware that the cannon has a very short range, so be ready for nasty retaliation by any surviving unit members.
The Bane Wolf fist made its gaming appearance in Dawn of War 2 where it was the unique unit for Inquisitor Adrastia. Just like in tabletop, the Bane Wolf was excellent against infantry and provided great advantages over units entrenched within buildings and cover. It is even decent against light armored vehicles as the lack of proper armor means that the chemicals could easily burn through the vehicle. Unfortunately don't expect to do too well with the likes of main battle tanks and super heavies.
The Bane Wolf's Chem Cannon is still a potent TEQ melter: D6 auto-hits, wounding on 2+ if the target's not a vehicle and AP-3 means that you've got a good chance at landing all of those hits, stopped only by armor. The problem is that it's got only D1, meaning that you'll need to get very lucky to kill two Terminators in one turn. And forget about using them against things like Paladins: you just can't provide the firepower to take them out. Because of the Chem Cannon's identical range the Heavy Flamer is a good supplement which might net you that one additional wound you need, clocking in at 105 points. Banewolves are best taken in squadrons.
Sometimes there are no alternatives. Sometimes you just want to strap on your Power Fist and punch a dude through the chest. When all else fails there's the Devil Dog; fielding the unique Melta Cannon, once the only Blast weapon to have the Melta rule in the entire game. Nowadays it is perfect to vaporise anything with a 2+ save, tanks or Monstrous Creatures. Remember that the tears of a Grey Knight player whose entire unit of Paladins was just vaporised by a cheapo tank are considered a delicacy on /tg/. Bottle them and share them with the guys at your local game store. All in all, it's the closest the Guard has to a medium tank: an armored vehicle that sacrifices armor for speed and power. Use the speed to get it in position fast, and the power to make back its points.
The Devil Dog's Melta Cannon has thankfully been turned into an Assault weapon as per the Codex, meaning that you can hit D3 times with a standard melta profile, and rerolls of your DD6 damage at half range. This means that you've got a decent shot at taking down all but the toughest of tanks in a single blast, which is a ton of fun. It's also semi-reliable against tough units of TEQs, but the low rate of fire is an issue when facing units of at least 5 models large. While it seems logical to take the Multi-Melta to supplement this, consider that it's a Heavy weapon, the Devil Dog has no way to mitigate this and it has a BS of 4+ at best. This configuration cranks the cost up to a worrying 128 points, which is quite a lot for a unit that'll likely end up getting taken out of the battle in a single turn.
Overall, all the tanks have access to the Heavy Bolter as well, and while it suffers from the same issue with it being Heavy, it is also the cheapest of the secondary guns. And the difference is notable: the Heavy Flamer is more than double the cost and the Multi-Melta is two and a half times its price tag. This means that a consideration has to be made: the cheaper and longer-ranged (longest range on the tank, mind you!) but less reliable Heavy Bolter, or the more expensive and complimentary but short-ranged Heavy Flamer. The Multi-Melta has the worst of both worlds: while its range is not the issue the cost, the different preferred targets, low accuracy and rate of fire means it's not a weapon you take just in case.
The trick to using these tanks is getting close to your preferred target ASAP and blast them to bits. The high speed and the smoke launchers (once per game you don't shoot, but instead invoke a -1 pentalty to hit with shooting. After that you'll likely get blasted into scrap, but if not you might get lucky a second time. Because they are cheap they make for ideal distractions as well, either with the threat of violence or as a physical screen for your other units.
With Power Ratings some of the issues are solved, but not all of them. The Heavy Flamers are now a lot more viable and while Devil Dogs cost the same 5 points as the other tanks, the Multi-Melta still suffers from the same problems. 5 points is still cheap, and can make for some highly mobile (if fragile) heavy weapons fire where your Heavy Weapons Squads cannot easily reach them. Overall they are useful, but not the best unit on your roster.
The term "Devil Dog" comes from a nickname of the U.S.M.C. allegedly given to them by the Krauts during World War One ("Teufels Hund", literally Hell Hound).