The following section contains information from a previous version of StarCraft II: Heart of the Swarm which is no longer valid.
Replicants were protoss-designed microscopic robots capable of fusing together to copy enemy units. They were created by the protoss in the aftermath of the first phase of the Second Great War. After analyzing the battles they had taken part in, replicants were created to better improve their war machine.
Replicants were floating ground units. Apart from massive units, they could replicate almost any unit type, including flying units and workers. When they did so, they also managed to copy upgrades (e.g. cloaking), even if they had not been researched. They could use their ability on any enemy unit on the map that was within a protoss player's line of sight. The drawback however, was that replicants were very expensive to create.
It was cut from development by April 2012, as it caused players to stop building certain units to keep replicants from copying them. Allen Dilling has expressed a hope that it will make an appearance eventually however, such as in the campaign mode and/or in Legacy of the Void. The replicant remains an unused unit within Legacy of the Void, but can be found as a functioning unit within the Galaxy Editor.
During development, replicants were once called "replicators."
The replicant's model appears to have been reworked for the disruptor. Side-by-side comparison using the Galaxy Editor will reveal the differences, primarily the color schemes and the size differences of the units (i.e. the Replicant is smaller and colored as shown above, while the Disruptor is larger in size and carries a Purifier color scheme).