This is a revamp of an earlier suggestion which has more merit now that replacement creatures are being grasped at Redcaps are the nastier and bloodthirstier (indeed downright psychotic) cousin of the goblin within folklore. This suggestion is, I should hope, very much altered in balance and positioning, to my original which i never really got around to refining.
The best image I could find.
The Redcap is an early-game/mid-game blitzer which relies on speed and gains small health and armour boosts from the deaths of surrounding enemies. Slightly mad, and certainly eccentric, Redcaps do not yield to reason and charge fearlessly into any fray without reservation or restraint. They are supremely quick and despite their relative frailty can deal a punch, hacking away with their twin short-handled axes in a mad frenzy.
On the flip-side they are physically weak creatures, carried into battle only by their overwhelming lust for blood. They have no sense of right and wrong, no conscience or moral compass - as such they are on occasion accompanied by sprees of petty theft, upsetting other creatures or causing fights - usually if they haven't seen battle for a long time. They may even steal from you, but this probably won't matter because for each creature they kill between paydays, they take 20% less gold.
Additionally, killing puts them in a state of 'Bliss' meaning they don't get angry or unhappy for a short while afterwards. They get on well with the undead, and have no whatsoever qualms being around them.
Health: Low
Attack: Medium-High
Armour: Low
Speed: Medium-High
Accuracy: High
Intelligence: Low/Moderate
Training Rate: Moderate
Training Cost: Above Average
Ease of Keeping: May upset others on occasion, but if they see enough combat can be very inexpensive and easy to please.
Recovery Rate: Medium
Creature Wage: Medium (The More Kills it Makes the Less it Takes)
Prospective Spells
Level 1 - Melee Attack
This is the standard close quarters melee attack.
Level 1 - Blood Healing (Passive)
This allows the Redcap to regain 5% of its total Health from kill nearby as well as temporarily buffing Attack Rate for about 3 seconds following and Speed for 10 seconds. The buffed Attack Rate and Speed don't stack, however.
Level 6 - Battle Frenzy (Passive)
This allows the Redcaps to regain another 5% of its total Health for every creature it personally kills.
History
Redcaps are among the most brutal, soulless and chaotic creatures to inhabit this underworld. It is quite frankly little wonder they work for you. These ghastly creatures are infamous for their horrifying tendency to stain their unkempt dread-locked hair red with the blood of their foes. They wear heavy armoured boots, which defy their furious swiftness, and a blood-splattered smock/tunic.
They are on whole solitary wandering creatures, congregating only to slaughter things and occasionally, if there is any time left over, procreate. As a result it is a inexplicable wonderment that they haven't killed themselves off already.
Images

The best image I could make. Also I forgot pointy ears, so add those in your head.
Old Redcap Suggestion
The Redcap is an eccentric, darker and, overall else, supremely blood-thirsty member of the the Goblin family. They very much lack the tell-tale cowardice of their lesser kin - and are never more at home than whilst on the battlefield.
The Redcaps gain health by killing and maiming their enemies. They also have a close affinity with the undead and feel very comfortable in their presence.
As units on the battlefield, Redcaps are supremely quick and very cunning.
Redcaps stain their dread-locked hair red with the blood of their enemies and carry tomahawks/short halberds.
The more creatures a Redcap kills the less gold it requires come payday.
Health: Low
Attack: Medium
Agility: Medium-High
Armour: Medium-Low
Speed: Medium/Medium-High (Speed Buffs Taken Into Account)
Accuracy: Very High
Intelligence: Moderate
Training Rate: Medium-Low
Recovery Rate: Medium (Also Recovers Health via Killing)
Creature Wage: Medium (The More Kills it Makes the Less it Takes)
Prospective Spells
Level 1 - Melee Attack
This is the standard close quarters melee attack.
Level 1 - Blood Healing (Passive)
This allows the Redcap to regain 25% of its total health from every kill it makes, it temporarily buffs attack rate and speed for a short amount of time following...
Level 3 - Carrion Attraction
This allows the Redcaps to teleport from one corpse or unconscious creature to a nearby one at random.
Level 8 - Blood Invulnerability (Passive)
This replaces the effects of Blood Healing and allows the Redcap to regain 50% of its total health from every kill it makes, it also temporarily buffs attack rate, speed and makes the creature invulnerable for a short amount of time following...