What They Are
There are three secret achievements. Two are for finding shortcuts, and one is for playing around.
How To Get Them
The first achievement, "Getting... Over It" requires you to take the upper path with only length 2. You can do this with the same tactic you do to get over the platforms at length 3.
The second achievement, "Head Over Heels" requires you to climb on top of the dragon's head. You can do this at length 3. You have to get on the horns.
The third achievement, "The Wiggler" requires you to take the lower path with only length 4. You can reach the ledge with the tip. After pulling yourself as far as you can, you have to do a wiggling motion, up and down, until you get through, like the thin tube section at length 5.
Closing
The game just came out a few hours ago, but I was able to find all the secret achievements very quickly. I tried to get the guide out as fast as possible, so please excuse any shortcomings.
Thanks,
Landon
Source: https://steamcommunity.com/sharedfiles/filedetails/?id=2511829864