Looter mercenaries are the mercenaries who have as favorite target: Resource Gatherers, Stockpiles and City Center. They mercenaries have high movement speed, low health and low damage. Night Rider and Sneak Peak are the only 2 common looter mercenaries in the game at this point that be hired at the Hiring Quarters.
The Samurai Team was introduced as a reward of the Plague Warriors event. Unlike the normal looters these Event items are deployed as a group of 12. The Samurai Team works just as any other looter mercenary.
With the 2013-07-26 update the Bearwolf also becomes a looter when it reaches level 6. His behavior was changed from "favorite target: non-utility" to "favorite target: Resource gatherers, Stockpiles, City Center".
With the 2013-08-03 update the Bearwolf behavior was reverted but it kept the looting behavior with other words it still has the 2.5 multiplier on hit.
Looting mechanics of Looter mercenaries
All units loot on hit and they loot each hit 0.45 times the damage dealt. The difference between other mercenaries and looters is that looter mercenaries loot 10 times the damage dealt on hit, this basically means they loot avarage 20 times more loot than all the other mercenaries.
The thing is looters only excels if they are attacking Stockpiles and City Centers whereas they can loot a higher percentage of the total loot compared to the other mercenaries, with resource gatherers this is not the case because you can only loot what is available in those resource gatherers and this is the same for all mercenaries.
Night Rider, Sneak Peak, Samurai Team and Bearwolf
In the 2014-08-04 update the looters received a nice buff and are even better at looting.
On hit loot multiplier 10x (previously 2.5x)
Looters will now loot a lot more from Stockpiles and City Centers, this will increase the amount of loot player who uses looters can get significantly.