I have given goblins a wyvern [mount] creature but I never got to the point where they bring mounts to test it properly yet. Some of what makes a civ have access to a creature(especially the use good animals and use evil animals) are still a bit of a mystery to me.
[CREATURE:WYVERN]
[NAME:wyvern:wyverns:wyvern]
[TILE:'W'][COLOR:4:0:0]
[MODVALUE:4]
[LARGE_ROAMING][FREQUENCY:25]
[POPULATION_NUMBER:1:2]
[CLUSTER_NUMBER:1:2]
[LARGE_PREDATOR][EVIL][MOUNT][MEANDERER]
[FLIER]
[PETVALUE:60]
[GRASSTRAMPLE:0]
[PREFSTRING:stingers]
[BODY:HUMANOID_ARMLESS:2EYES:2EARS:NOSE:2LUNGS:HEART:GUTS:2WINGS:ORGANS:THROAT:NECK:SPINE:BRAIN:MOUTH:TAIL:TAIL_STINGER]
[BODYGLOSS:TALON_FOOT]
[SIZE:9]
[MAXAGE:10:20]
[ATTACK:MAIN:BYTYPE:MOUTH:bite:bites:1:6:GORE][ATTACKFLAG_CANLATCH]
[ATTACK:MAIN:BYTOKEN:TAIL_STINGER:sting:stings:1:6:PIERCE][SPECIALATTACK_INJECT_EXTRACT:50:100]
[CHILD:1][CHILDNAME:wyvern hatchling:wyvern hatchlings]
[FAT:1]
[EXTRACT:wyvern venom:7:0:0]
[EXTRACT_VALUE:100]
[EXTRACT_PARALYZE]
[EXTRACT_ANTIDOTE:wyvern antivenin:7:0:0:100]
[ALL_ACTIVE]
[BIOME:NOT_FREEZING]
[BIOME:MOUNTAIN]
[STANDARD_FLESH]
[HOMEOTHERM:10067]
[SWIMS_INNATE][SWIM_SPEED:2500]
If ya want any old goblin to be able to fly over walls though just give them [FLIER]. Doubt you even need to regen the world. Call them the wicked witch's flying monkies or something.
Well if five days, three hours, and 2-3 minutes of lurking on this forum have taught me. A civilization will typically have materials, and animals depending on their location, so really having your goblins settle exactly on the same place as wyverns, asuming the animals don't totally kill them, and are probably trainable. They may tame them, and use them as war animals, you could also check the beak dogs profile.