Specifically, I am looking for a list of creatures yielding horned leather hides but ideally would like lists of other things too, like scales, etc...

The hunter guide on stratics is your best friend

Normal : cows, goats, hinds, harts, bulls, hiryus, etc.
Spined : Lizardmen are your friends, alligators and some other swamp creatures or giant serpents if I'm not mistaking.
Horned : Sea serpents (+scales), Wyverns, Drakes (+scales), some of the lava animals if I remember well and I believe the silver serpents.
Barbed and scales : Dragons, greater dragons, wyrms (all of them)
Bones : undeads, shadow wisps.
Feathers : harpies, birds.
Wool : sheeps (better get the wool when they're alive)

I think that's pretty much it when it comes to ressources from monsters.

swamp dragons give normal
ratmen give spined

rest of the list is correct

Right. The swamp dragon gives green scales but normal hides.

Thanks guys!

Which leads to my next question: Are wyverns and drakes tough to kill and what's the best place to get them? I've been doing sea serpents and they are a piece of cake.

Wyverns deal greater poison, which is not fun unless you kill them with earthies/EVs.
The best spawn of Wyvern is ... wyvern island in the Lost Lands, north of Papua in the sea.

Drakes are piece of cake, though slightly tougher than sea serpents. Bad news is, no spawn of Drakes only. Either destard and you'll meet the Greater Dragons ;-) Or Valor forest and you'll meet paragon wyverns and drakes. Or some of the other Dragon spawns.

If someone know any easy spot... I'm eager to know them! It'd be good info to post on my "how-to-make-gold" article

Wyvern island sounds interesting, I'll have to check that out. Trying to put together a Felucca resource collecting runebook with minimum risk of death

Risso,

personally just find or mark a rune to wyvern island in trammel. Why? Because they spawn rather quick there. No bother from pks and with the bag of sending changes your going to be recalling to your house/bank anyway. While double resources are great. You just do not need them if your gathering for yourself. It takes my pally about 7-10 minutes to fill up with cut horned leather. If I take the gold too, I general get about 1000 hides in about every 30 minutes. Even at this rate, I could turn around and sell 1000 hides for about 40-45k. Not bad for really little work.

How to make gold article??

I'd like to add that drakes are also found at the temple on fire island and in the dungeon below the terathan keep. But those are places with a lot of nasty spawn to deal with.
my fav place to hunt drakes outside ilshenar has always been Coveteous lvl 5. Sadly it's been occupied by that outdated quest-dragon now...