Imp
ImpHealth: 4Attack: 2Shots: noneMovement: ground
50 gold (15)Hex size: 1Defence: 3Dmg: 1-2Spd: 5 (slow)
Weakest level 1 unit in Heroes 3 which is also quite expensive. Useless all around :)
 
Familiar
FamiliarHealth: 4Attack: 4Shots: noneMovement: ground
60 gold (15)Hex size: 1Defence: 4Dmg: 1-2Spd: 7 (extra swift)
Special: 20% of mana spent by enemy hero on spells will be chanelled into your hero's mana pool.
Upgrade is well worth doing: attack and defence become closer to medium for level 1, good speed and the mana chanelling abbility which makes it somewhat worthy having familiars present at the long battles. Think about it: he spends 20 mana and you'll get 4! Damage range does not reach 3 which is a big disadvantage. If you get the grail, you can get hordes of these guys :)
 
Gog
GogHealth: 13Attack: 6Shots: 12Movement: ground
125 gold (8)Hex size: 1Defence: 4Dmg: 2-4Spd: 4 (extra slow)
Very good shooter, nice damage range and hit points are a meaningful number to them. Speed is too slow before upgraded though.
 
Magog
MagogHealth: 13Attack: 7Shots: 24Movement: ground
175 gold (8)Hex size: 1Defence: 4Dmg: 2-4Spd: 6 (swift)
Special: ranged attack also affects adjacent hexes with fire damage.
Magog cannot beat marksman's two shots, but if enemies are standing close together, the advantage can be even grater. Note that fireproof units do not suffer magog's adjacent damage.
 
Hell Hound
Hell HoundHealth: 25Attack: 10Shots: noneMovement: ground
200 gold (5)Hex size: 2Defence: 6Dmg: 2-7Spd: 7 (extra swift)
Good offensive unit, low on defence. Strongly affected by bless and curse spells.
 
Cerberus
CerberusHealth: 25Attack: 10Shots: noneMovement: ground
250 gold (5)Hex size: 2Defence: 8Dmg: 2-7Spd: 8 (very swift)
Special: can attack 3 adjacent enemies at once, no enemy retaliation.
Great upgrade. This is the only case in the game where stats get downgraded, damage in this case because this damage can be done to 3 enemies at once. Better defence and with no retaliation it's a good idea to charge and attack the crowd.
 
Demon
DemonHealth: 35Attack: 10Shots: noneMovement: ground
250 gold (4)Hex size: 1Defence: 10Dmg: 7-9Spd: 5 (slow)
Average unit for level 4, but the price is very reasonable.
 
Horned Demon
Horned DemonHealth: 40Attack: 10Shots: noneMovement: ground
270 gold (4)Hex size: 1Defence: 10Dmg: 7-9Spd: 6 (swift)
One of the smallest upgrades in the game: 5 hit points and 1 speed for 20 gold... reasonable, but horned demons are now below-average among level 4 upgrades, but their price still remains low.
 
Pit Fiend
Pit FiendHealth: 45Attack: 13Shots: noneMovement: ground
500 gold (3)Hex size: 1Defence: 13Dmg: 13-17Spd: 6 (swift)
Not a very good level 5 creature, but costly. Demons and pit fiends kinda balance each outher out. Note how often number 13 is used :)
 
Pit Lord
Pit LordHealth: 45Attack: 13Shots: noneMovement: ground
700 gold (3)Hex size: 1Defence: 13Dmg: 13-17Spd: 7 (extra swift)
Special: Can resurrect dead allied stack as demons once per combat. Number of raised demons cannot be larger than the number of pit lords performing the resurrection. Max number of demons is determined by the total hit points of the target stack.
1 speed and demon resurrection abbility for 200 gold! Sounds darn expensive. Think this way: if you have lost 90 imps on a battle, 10 pit lords can resurrect them into about 10 demons... or if you lost 14 hell hounds, 10 pit lords can also turn them into about 10 demons. To raise 50 demons you will need 50 pit lords and a dead stack of about 70 hell hounds or 20 efreet or 440 imps. 50 Pit lords will cost 10000 gold to upgrade. 50 Demons cost 12500. Think for yourself... By the way, due to special-only upgrade, pit lords are weak among level 5. If you're low on money, pit lord costs almost like 3 demons who will make a stronger force.
 
Efreeti
EfreetiHealth: 90Attack: 16Shots: noneMovement: flying
900 gold (2)Hex size: 1Defence: 12Dmg: 16-24Spd: 9 (ultra swift)
Special: Immune to fire, 150% damage vs genies.
Reasonable creature, can be purchased early due to inferno's specific building plan. Good troop for armageddon spellcaster.
 
Efreet Sultan
Efreet SultanHealth: 90Attack: 16Shots: noneMovement: flying
1100 gold (2)Hex size: 1Defence: 14Dmg: 16-24Spd: 13 (very quick)
Special: Immune to fire, 150% damage vs genies, have fire shield spell on.
Excellent upgrade. Their speed is only matched by some level 7 upgrades and... dragon flies. Enemy will think well before attacking efreet sultans: portion of the damage returns back to the attacker plus the retaliation. Good to use them against high level creatures because fire shield damage will surpass their mighty defence without being decreased.
 
Devil
DevilHealth: 160Attack: 19Shots: noneMovement: teleport
2700g+1m (1)Hex size: 1Defence: 21Dmg: 30-40Spd: 11 (quick)
Special: no enemy retaliation, 150% damage to angels, -1 enemy luck while present on the battlefield.
Makes a tough opponent because of speed and no enemy retaliation. Devil can beat an angel in one on one combat.
 
Arch Devil
Arch DevilHealth: 200Attack: 26Shots: noneMovement: teleport
4500g+2m (1)Hex size: 1Defence: 28Dmg: 30-40Spd: 17 (extra fast)
Special: no enemy retaliation, 150% damage to angels, -1 enemy luck while present on the battlefield.
Arch devil is no match for an arch angel. Otherways, a really powerful creature for non-retaliated attacks with outwaiting the opponent.