OK, this card is somewhat decent from looking at it already.
90 HP is great for a stage 1. Fighting weakness is a pain, the psychic resistance is fine, and the retreat cost is average.
Houndoom's first attack seems to make up the fighting weakness. For only one dark energy, you can do 20, but if the opponent has any :fighting: Pokemon out, then it will do 80. You can actually increase the damage with special darks and Expert Belts, so that seems pretty interesting. Its second attack , for two darks, does 50, and your opponent discards a card from his or her hand. Again, there are ways to make the attack even more effective, and since it offers some disruption, that does make it kind of usable.
Overall 3/10 in modified, 8/10 in modified, and 2/10 in unlimited
In modified, to be honest, this card is a bit below average. Sure it can hit for some high damage if any :fighting: Pokemon around, and it can cause some nice disruption, but there are ways this card can be countered now. Regardless, a Donphan can easily sweep a Houndoom, as well as many other good :fighting: cards, like Machamp. The HP isn't that convincing too. Thus, I don't think its that good right now, but it could be good for fun.
In limited, this card is actually pretty good, and to be exact, this is where this card shines. There aren't that many real good fighting types in UD, so Fire Counterattack has the potential to hit for some nice damage, and so does Dark Roar. Comboed with special darks, you can really be hitting for some pretty good damage. Moreover, Dark Roar can be annoying for the opponent since it forces them to discard a card from their hand. Indeed, Houndoom can make a nice addition to any :dark: deck here.
In unlimited, lets face it, this card isn't going to do a lot. Again, it has some nice attack coverage overall, but the HP is a bit lacking in a format like this, and many good fighting types can take it down easily. Despite this, I still see some nice use with this card here, especially when against the not-so-good fighting types.