Ambulocetus, the Walking Whale

Large beast, unaligned

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Armor Class: 14 (Natural Armor)
Hit Points: 102 (12d10 + 36)
Speed: 30 ft., swim 40 ft.

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STR	DEX	CON	INT	WIS	CHA
19 (+4)	14 (+2)	17 (+3)	3 (-4)	14 (+2)	6 (-2)

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Saving Throws: Str +7, Con +6
Skills: Perception +5, Stealth +5 (in water)
Senses: Darkvision 60 ft., Passive Perception 15
Languages: —
Challenge: 5 (1,800 XP)

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Traits

• Amphibious. Ambulocetus can breathe both air and water.
• Keen Smell. Advantage on Wisdom (Perception) checks that rely on smell.
• Water Camouflage. While submerged, Ambulocetus has advantage on Dexterity (Stealth) checks.

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Actions

• Multiattack. Ambulocetus makes two attacks: one with its Bite and one with its Claw.
• Bite. Melee Weapon Attack: +7 to hit, reach 10 ft., one target.
  Hit: 17 (3d8 + 4) piercing damage. The target must succeed on a DC 15 Strength saving throw or be grappled (escape DC 15). While grappled, the target is restrained, and Ambulocetus cannot bite another target.
• Claw. Melee Weapon Attack: +7 to hit, reach 5 ft., one target.
  Hit: 12 (2d6 + 4) slashing damage.
• Drag Prey (Recharge 5–6). If Ambulocetus has a creature grappled with its Bite, it can drag the target up to 20 feet into water or swamp terrain. The target must succeed on a DC 15 Strength saving throw or be restrained and begin to drown (see Drowning condition).

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Legendary Actions

Ambulocetus can take 1 legendary action at the end of another creature’s turn. It regains spent legendary actions at the start of its turn.

• Lunge. Ambulocetus moves up to its speed without provoking opportunity attacks.
• Water Slam (Costs 2 Actions). Ambulocetus slams its tail into the water, creating a shockwave in a 15-foot cone. Each creature in that area must succeed on a DC 15 Dexterity saving throw or take 10 (3d6) bludgeoning damage and be knocked prone.

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Reactions

• Snap Defense. When a creature misses Ambulocetus with a melee attack, it can use its reaction to make a Bite attack against that creature.

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Tactics

Ambulocetus lies in wait submerged near shores and riverbanks, striking unsuspecting prey that approach the water’s edge. It uses stealth and surprise to grapple targets with its powerful bite, dragging them underwater to drown. On land, it relies on claw attacks to fend off threats but is most lethal in water where it can maximize mobility and stealth. It uses Water Slam to disrupt groups of attackers, then isolates and grapples foes to finish them.

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Drowning (Optional Homebrew Condition)

• A creature grappled underwater must hold its breath or start suffocating.
• At the end of each of its turns, it makes a DC 15 Constitution saving throw.
• On failure, it takes 10 (3d6) damage. On reaching 0 HP, it suffocates.
• On success, it resists damage but remains restrained until it escapes.

Ambulocetus

CR 6
XP 2,400
Male large animal
Init +2; Senses darkvision 60 ft., low-light vision, scent 30 ft.; Perception +10

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DEFENSE
AC 17, touch 11, flat-footed 15 (+4 natural, +2 Dex, +1 size)
HP 68 (8d8+32)
Fort +9, Ref +6, Will +4

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OFFENSE
Speed 30 ft., swim 40 ft.
Melee bite +11 (2d6+6 plus grab), 2 claws +11 (1d8+6)
Special Attacks improved grab, drag prey, drown

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STATISTICS
Str 22, Dex 15, Con 18, Int 2, Wis 14, Cha 6
Base Atk +6; CMB +12 (+14 grapple); CMD 24 (27 vs. grapple)
Feats Improved Grab, Multiattack, Power Attack, Weapon Focus (bite)
Skills Perception +10, Stealth +8 (+12 in water)
Languages —
SQ amphibious, keen smell, water camouflage

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SPECIAL ABILITIES

Amphibious (Ex): Ambulocetus can breathe air and water.

Keen Smell (Ex): +4 bonus on Perception and Survival checks relying on smell.

Water Camouflage (Ex): While submerged in water or swamp terrain, Ambulocetus gains a +4 bonus on Stealth checks.

Improved Grab (Ex): To use improved grab, Ambulocetus must hit with a bite attack. It can then attempt to start a grapple as a free action without provoking attacks of opportunity.

Drag Prey (Ex): While grappling a creature with its bite, Ambulocetus can move at half speed, dragging the target into water or swamp terrain. The target must succeed on a CMD 20 check or be dragged against its will.

Drown (Ex): A creature grappled and dragged underwater must hold its breath or begin suffocating, taking 1d6 damage per round while restrained underwater.

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ECOLOGY
Environment warm coastal lagoons, river deltas, marshy estuaries
Organization solitary or pair
Treasure none

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DESCRIPTION
Ambulocetus is a large, semi-aquatic predator that blends the bulk of a prehistoric whale with the agility of an otter. Covered in thick fur, it lurks near shores, stalking prey with patience and deadly stealth. It uses its powerful jaws to grapple and drag prey into the water to drown. On land, it relies on its claws and brute strength to defend itself but prefers to retreat to the water where it is most effective.

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TACTICS
Ambulocetus relies on stealth, striking from underwater ambushes with a powerful bite and immediately attempting to grapple and drag its prey into deeper water. It uses Power Attack to maximize damage during grapples and claw attacks to fend off threats on land. It avoids prolonged fights on land and uses its superior swimming speed to control the battlefield.

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Ambulocetus

The creatures resting on the bank are quite unusual, looking like furry crocodiles.

Originally Posted by Raptorial of the Wizards Community forums. 
On this Thread 

Ambulocetus is an early cetacean that can walk as well as swim. Having the appearance of a three-meter long mammalian crocodile, it was clearly amphibious, as its back legs are better adapted for swimming than for walking on land. Ambulocetids hunt like crocodiles, lurking in the shallows to snatch unsuspecting prey. They are able to move between salt and fresh water, haunting coastal waters and forest waterways.. 

Ambulocetus
Large animal
Hit Dice	3d8+21 (34 hp)
Initiative	+2 (+2 Dexterity)
Speed	15 ft. (3 squares), Swim 30 ft. (6 squares)
Armor Class	13 (-1 size, +2 Dexterity, +2 natural), touch 11, flat-footed 11
Base Attack/Grapple	+2/+14
Attack	Bite +9 melee (1d8 +8)
Full Attack	Bite +9 melee (1d8 +8)
Space/Reach	10 ft./5 ft.
Special Attacks	Improved grab
Special Qualities	Low-Light Vision
Saves	Fort +10, Ref +7, Will +4
Abilities	Strength 26, Dexterity 15, Constitution 25, Intelligence 2, Wisdom 16, Charisma 9
Skills	Hide +5, Listen +3, Move Silently +9, Spot +4
Feats	lightning reflexes, Stealthy
Environment	Warm coast or forest rivers
Organization	Solitary
Challenge Rating	3
Treasure	None
Alignment	Always neutral
Advancement	4-6 HD (Large); 7-9 HD (Huge)
Level Adjustment	–

Combat

An ambulocetus will bite an opponent and attempt to drown it.

Improved Grab (Ex) If an ambulocetus bites an opponent, it deals normal damage and attempts to start a grapple as a free action without provoking an attack of opportunity. No initial touch attack is required. Improved grab works only against opponents that are Medium or smaller. The ambulocetus may conduct the grapple normally, or use its bite to hold the opponent. If it chooses to do the latter, it takes a –20 penalty on grapple checks, but is not considered grappled itself; it doesn’t lose its Dexterity bonus to AC, still threatens an area, and can use its remaining attacks against other opponents.

Each successful grapple check it makes during successive rounds automatically deals bite damage. When the ambulocetus gets a hold after an improved grab attack, it pulls the opponent into its space. This act does not provoke attacks of opportunity. The ambulocetus can even move, along with the opponent, if it can drag its weight.

Hold Breath (Ex) An ambulocetus can hold its breath for a number of rounds equal to 7 x its Constitution modifier.