Golem is a Unit.
How to Get Edit
Golem is noncollectible and can only be generated by effects.
A Golem is created when a spell or effect needs to create or transform a unit, but no units with its power cost currently exist. Its attack and health stats are equal to its power cost.
For example, playing Unstable Form on a 10-cost unit, such as Scourge of Frosthome or The Tormentor, will transform it into an 11-cost 11/11 Golem. (Note: At the time of writing no 11-cost units exit. This may change as new cards are released.)
If you Unstable Form a Golem with no higher-cost unit, it will become a new Golem with +1/+1 and its cost increased by 1.