Here is the full list of all 51 Kukoos - there are Lost Pets achievements.
Fall into abyss 20 times
Free 20 Coaches
Complete World 1
Use 50 flashes
Finish a level without taking damage
Free 100 Pets
Collect 10 Fwendly plants
Collect 1000 Coins
Collect 100 Kukoonuts
Get 3 stars in one level
Collect 5 Fwendly plants
Free 50 Pets
Collect 500 Coins
Free 20 Stoneys
Collect your first Fwendly plant.
Free 10 Pets
Free your first pet
Collect 100 Coins
Complete the Pet Day trial
Collect 2500 Coins
Collect 5000 Coins
Collect 30 Fwendly plants
Collect all the Fwendly plants
Get 3 stars in 5 levels
Get 3 stars in 10 levels
Get 3 stars in all levels
Complete World 2
Complete World 3
Complete World 4
Defeat all four sea creature bosses
Defeat the Frog
Use 100 Stomps
Shoot 500 projectiles with Kurabo
Turn 100 times into a ghost
Flap Petri wings 500 times
Free 20 Funicorns
Free 20 Pizzaiolos
Make a 3 hit jump combo
Free all pets in a single level
Free all pets of the game
Beat the game in under 3 hours
Make a 5 hit Kukoonut break combo
Get 10 Powerups
Free 10 centipedes hitting only the head
Complete a boss without losing life
Get 50 Checkpoints
Complete 100% of the game
Drown 20 times
Get 10 player kukomands
Die 50 times
Die 10 times in a boss