Here is the full list of all 12 Milly the dog achievements.
Milly has completed stage 1
Milly has completed stage 2
Milly has completed stage 3
Milly caught more than 100 balls
Milly caught more than 100 flying disc
Milly driven the cattle to the ranch in less than 10 seconds
Milly has been cleaned very fast
Milly has completed the agility course in less than 45 seconds
Milly has completed race track in less than 30 seconds
Milly caught more than 100 stars
Milly has won all minigames achievements
Milly has more than 1000000 XP points