Retaliation
Retaliation refers to the counterattack of a stack that has been the target of a melee attack. All creatures have one retaliation per combat round with the exceptions of several creatures: Griffins, which have two retaliations per combat round; Royal Griffins and Fangarms (available in the Horn of the Abyss expansion), which have an unlimited number of retaliations per combat round. With the Counterstrike spell, it is possible for a hero to magically add one or two extra retaliations per combat round. For Griffins this means three or four in total.
Some creatures have special ability typically phrased as "no enemy retaliation", which does not allow the stack(s) they attack to retaliate against them. This also means, that the attacked stack will not lose one of their retaliations for that combat round if attacked by a creature stack with no enemy retaliation. Thus, while attack with no retaliation is a very strong ability, sometimes it's a double-edged sword as creatures with no enemy retaliation cannot be used to prevent an enemy creature from retaliating that round.
Creatures with no enemy retaliation:
Naga and Naga Queen
Cerberus
Devil and Arch Devil
Vampire and Vampire Lord
Harpy Hag
Hydra and Chaos Hydra
Sprite
Psychic Elemental and Magic Elemental
Corsair and Sea Dog 
Sentinel Automaton
Creatures with more than one retaliation per combat round:
Griffins have two retaliations.
Royal Griffins have unlimited retaliations.
Fangarms have unlimited retaliations.
Creatures with special attacks that do not cause retaliations:
Crimson Couatls can use the invulnerability provided by Meditation to attack without retaliation for one turn per battle.
Dreadnoughts and Juggernauts can use heat stroke instead of attacking; this ability does not cause retaliations.
Creatures causing modified retaliation:
Unicorns and War Unicorns can 
Blind on attack which prevents creatures from acting and decreases damage of their first retaliation by 50%.
Medusas and Medusa Queens can stone gaze on attack which prevents creatures from acting. The next attack on that creature will do 50% damage.
Scorpicores can paralyze on attack which prevents creatures from acting. The next attack on that creature will only be retaliated against at 25% strength.
Basilisks and Greater Basilisks can petrify on attack which prevents creatures from acting. The next attack on that creature will do 50% damage.
Creatures with retaliation against ranged attacks:
Gunslinger has one preemptive shot except AoE ranged attacks: Liches' Death Cloud and Magogs' Fireball Attacks.
Bounty Hunter has unlimited preemptive shots except AoE ranged attacks.
