Goalby

Goalbies are melee mobs that spawn in the Extreme Hills Biome of the Overworld. It is immune to knockback and inflicts a powerful Weakness effect on hit. It yields a small amount of hunter experience when killed. It has the Attributes of Gun and Bow Immunity.

If you are playing the FTB Departed modpack (or just have Tinker's Construct with AoA), Tinker's Construct-based weaponry, such as the Cleaver or Longsword, can only do 1/2 heart of damage to it per hit. If your main sword is made using Tinker's Construct as opposed to a sword crafted in a workbench, you'll find hunting these to be difficult, unless you bring a sword that can more easily harm a Goalby.

This effect is believed to be a bug.

References/Sources
http://adventofascension.gamepedia.com/Goalby