Hellhound
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.
Warhammer 40000
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.
Overview
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.
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.
Variations
There are currently three patterns of the Hellhound in use: the Hellhound, the Bane Wolf and the Devil Dog. Like the Chimera, the Hellhound tank can field a hull-mounted Heavy Flamer or Heavy Bolter, and can also field a Multi-Melta.
Hellhound
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.
This is by far the most useful variant, it gets to fire its template 12 inches out, then the template itself, meaning on first turn of 12 inch movement + 12 inch range + flame template = first turn roasting. This means you're never in actual danger of being assaulted unless it gets surrounded or out maneuvered. Have fun.
Bane Wolf
When you want to fuck up MEQ. The Bane Wolf Chem Cannon is a Str 1 Poisoned (2+) Ap3 weapon, meaning that it 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 only has template range, so be ready for nasty retaliation in the form of melta(bombs) by any surviving unit members. Because of the nature of its main weapon the Bane Wolf gets more mileage out of its secondary weapon, making the Heavy Bolter a prime choice when you want your Bane Wolf to at least be able to defend itself at a longer range.
Devil Dog
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 (although there is near half dozen of them now). Though this means that it has a bizarre roll against armor (8+2d6 against vehicles within short range, 8+1d6 at long range), it is perfect to vaporise anything with a 2+ save or multiple wounds without Eternal Warrior. Remember that the tears of a Grey Knight player whose entire unit of Paladins was just vaporised by a 130 point tank are considered a delicacy on /tg/. Bottle them and share them with the guys at your local game store.
While the short range of its weapon makes it something of a difficult to use beastie, remember that it is a Fast vehicle and as such can zoom towards the enemy at high speed to get a good shot in. After getting that shot off however it'll be a sitting duck for any unit with at least a modicum of anti-tank weapons (read: lots of them), making it likely that the Devil Dog(s) are blasted apart afterwards. Also note that while its weapon can be used against Monstrous Creatures and other such powerful units it is more effective against vehicles and squads of TEQ units because of its effectiveness against armor; the Devil Dog can reliably deal wounds, but only one of them at a time.
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, because the armor won't save it against the reprisal.
Trivia: The term "Devil Dog" comes from a nickname of the U.S.M.C.