Chickencow

The Chickencow can be tamed as a mount by rolling successful Animal Handling checks. One check per week. This check since it represents a 7 day time period may not benefit from guidance, bardic inspiration, assists, or any other external means of enhancing the skill check. The initial DC is 15, and if any of the 4 checks fail, then the process must be repeated and the DC is increased by 1. During this period, any stress placed on the animal from lack of food, water, environmental, or physical discomfort requires a straight DC10 check to determine if the creature becomes impossible to tame. If this should occur, the creature will wait until its would-be tamer is within striking distance and attack. 

Raising a Chickencow from an egg requires the egg to be buried in cow manure for 4 weeks. Once hatched the Chickencow Chick requires 10 pounds of raw meat from any source per day for 4 weeks. If this is completed successfully, then the former Animal Handling check must be completed as described above, however, the initial DC drops to 10.

The Chickencow's standard attack is 1 Beak and 2 Hoof attacks per round. These attacks will be focused on the same target even if a previous attack has killed or incapacitated the target.

Beak. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 18 (3d8 + 4) slashing damage.

Hoof (x2). Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 10 (1d12 + 4) bludgeoning damage. DC15 vs Strength or be knocked Prone.

Flailing Hoofs. If an enemy passes within 5 feet of the hind legs of the Chickencow it will use its reaction to make a single hoof attack against the target. On a successful hit the target is knocked prone, no save, and must make a DC15 save vs Constitution or be stunned for 1d4 rounds. If an enemy is laying prone next to the Chickencow at the start of its turn, it will use its reaction to trample the target; hoof attack x 4.

Crazed Peck. If there is an enemy in stealth within 20' of the Chickencow, and the Chickencow percieves it, it will stampede toward the stealthed enemy and perform a single Beak attack.  All entities (even incapacitated ones) that are,  or become,  adjacent between the Chickencow and the target are subject to a Hoof attack. Attacks of opportunity against the Chickencow are at disadvantage, and can only be made if the player has not been struck by a hoof.

Flying Charge. When attacked from range within 120', the Chickencow can fly at the attacker and execute a Beak attack and (4) Hoof attacks.

Really Pissed Off Squawk. When struck by a melee weapon the Chickencow will squawk. All non-Chickencow entities within a 60' radius must save vs Wisdom DC15 or be stunned for 1d4 rounds.

Flatulence. The Chickencow can produce a gaseous emission that lingers in a 30' radius for 1 round. Any non-Chickencow entities within the gas must save DC15 vs Constitution or suffer one level of exhaustion. The gas has been known to cause disease.