I have tried spawn rates of 10, 100, 1000, and 90 in Extreme Hill Biomes. I have also tried at night and during the day, as well as underground caves. I have tried multiple different Forge Versions as well, with no luck of seeing this mob. I would be glad to do a video on it tomorrow once I figure this out, lol.
Strange. I tweaked the spawns to forests, so I may have them spawn there soon. If it is a bug I'll push out a new build tonight