For the Dire Hyena's pulling wagons - add
[COMMON_DOMESTIC_PACK]
[COMMON_DOMESTIC_PULL]
[COMMON_DOMESTIC_MOUNT]
[COMMON_DOMESTIC_PET]
to your Gnoll's entity file and they should use them for wagons, if you want to stop Dwarves and Humans using them you can add
[EVIL]
to the Dire Hyena's creature file and
[USE_EVIL_ANIMALS]
to the Gnoll's entity file
Although this means that Goblins may take them along with Beakdogs when they siege, not sure on that though. If you can live without Beakdogs with goblins you can remove the [USE_EVIL_ANIMALS] from the Goblins file - I think that should fix it, if that is actually a problem. I think you can also add [PACK_ANIMAL] to the Dire Hyena for it to pull merchants wagons.
On rotting meat, I don't think that it can be eaten, I believe it is hard-coded, although I may be wrong on that point.
For leather as a building material, you might be able to make a separate stone that doesn't appear in any layers and make a furnace reaction to change leather into the 'leather stone', as I don't think you could make the leather itself work as a building material.
Also you might want to change the Gnoll's prefstring, as the current 'Ugly' means that if you get a migrant who likes Gnolls I think it would say something along the lines of 'Likes Gnolls for their ugly', which sounds somewhat strange.
You also have
[RELIGION_SPHERE:SPHERE_DEATH]
[RELIGION_SPHERE:SPHERE_CHAOS]
[RELIGION_SPHERE:SPHERE_REVELRY]
[RELIGION_SPHERE:SPHERE_DEPRAVITY]
[RELIGION_SPHERE:SPHERE_HUNTING]
in the Gnolls religion, it should just be
[RELIGION_SPHERE:SPHERE_DEATH]
[RELIGION_SPHERE:CHAOS]
[RELIGION_SPHERE:REVELRY]
[RELIGION_SPHERE:DEPRAVITY]
[RELIGION_SPHERE:HUNTING]
A slight spelling issue [ETHIC:ASSAULT:ACCETABLE] (Needs a 'p')
[LEADER_TYPE:MATRIARCH] means nothing, the leader type has to be taken from this list =>
http://dwarffortresswiki.net/index.php/Profession_tokensAnd inside the matgloss_gnoll_steel.txt file, you have the header 'matgloss_
metal_gnoll_steel' rather than 'matgloss_gnoll_steel'
You also have an empty text file - matgloss_metal_gnoll_steel.txt, perhaps this relates to the above issue?