I think I'll pm Rysith *the creator of the orc mod* to see how he linked creatures to the orcs, I'm pretty sure it was the evil and mount tags but can't hurt to ask.
Replying in the thread, to spread the knowledge that I have.
In general, creatures with animals that have [MOUNT] will bring those along as mounts on sieges. Aligned creatures (with the [GOOD] or [EVIL] tags) can only be used by entities with [USE_GOOD_ANIMALS] and [USE_EVIL_ANIMALS], respectively. Civilizations will also only bring along animals that appear in their region of control, which I believe is a three-by-three world map tile area centered on each of their fortresses. This means that beak dogs in vanilla are fairly rare, since the goblin civ needs to build a fortress next to an evil swamp. By contrast, the Orc's elephants (evil grasslands and jungles) show up fairly commonly. A single fortress can supply mounts for the entire civilization, so expansive civs will be more likely to have mounts, as well.
So, to get a mount all you need to do is add a creature with the [MOUNT] tag and make sure that the civ can use it. Any siege you get will show up with either just the leaders or everyone mounted. Note that any other civ that can use the animal may use it as a mount too, so keep that in mind when setting the tags. For testing purposes, you can set the biome to [ANY_LAND] to let them show up anywhere, which helps. I'm not sure how they deal with multiple possible mount creatures, but I've only ever seen one kind of mount per civ, so removing other mounts might be a good idea during testing.
This makes me think that Trolls are hard-coded, since they don't have the [MOUNT] tag, but you never see goblins showing up with the other evil animals. The fact that they showed up alongside the goblins, not being ridden by goblins, also seems to point to that to me. I'll admit, though, the I've never seen a Troll show up in 3d, so that may have been made consistent and all that remains are stories from the elders.