Red Skeletons, also known as Mimics, are summoned after destroying a "fake" golden chest. In doing so, a skeleton will pop out, and after traveling a set distance of blocks, will die, dropping coins. They last longer and move faster the farther you go.
Between 0-100 the skeletons are basically free money due to how easy they are. 100-150 is somewhat harder. 150-200 is much harder and probably shouldn't be opened if you don't have an easy obstacle following it. Anything more than 220+ meters is a death sentence. The best strategy is to stand above the chest, use the Duke's Darkwing ability, open the chest and then dart upwards until the skeleton dies.
It is very important to note that not only do they last longer and run faster the further you are, but also have a faster reaction time. At 150+ make sure you have your path set before opening them and move as soon as you do.