Runite Golems are found in the Resource Arena east of the Deserted Keep. They are weak to melee, especially pickaxes. A higher mining level means more damage will be dealt. When it dies its head will temporarily become a runite rock, which can be mined with 85 mining. Once mined it respawns in 5 minutes. There are only two of them in each world.
Previously, a runite golem that has been poisoned or envenomed would stay infected even after being transformed into a mineable rock. This caused 1 damage, causing the rock to disappear and leave behind a set of bones. This glitch was fixed in an update on 23 April 2015.