The latest version (1.5.0) released last week added a few achievements, "Shoe Size: Monster", "Cryptozoology Breakthrough", "Mechanical Rowboat", "That About Wraps It Up" and "Meow Meow Meow!". They're intended to be secret, since there's no indication how to unlock them at all, except very, very vague hints about where you're likely to find them.
I only managed to unlock the last one because it's obvious to those who finished the game. But what about the others? How to unlock them?
Best Answer
For the sake of completeness, I'll just list all the new secret achievements.
Summer, Shoe Size: Monster (originally posted by andbeyond)
Winter, Cryptozoology Breakthrough
Future, Mechanical Rowboat
Pyramids, That About Wraps It Up
Cat, Meow Meow Meow!