Question: I have heard both interpretations, for me it would make more sense to be something from the mind as there is no creator God or its opposite in Buddhism, the image of a "temptator creature" like Satan sounds strange when we think about impermanence, on the other hand, some suttas that mention Mara may fit better with the concept of an external creature, I know this interpretation is questionable, so I'm making the question!

Answer: 
There are multiple [Maras](http://en.wikipedia.org/wiki/Mara_(demon)) in Buddhism. One of them is a Deva. Other others being death, conditioned existence, and unskillful emotions.
Using the Three Universal Characteristics of impermanence, suffering and non-self, you can see that "mara" is a creation of the mind, this is why "mara" is in or attached to everything!!

It is only a cultivation of wisdom ( The Noble Eightfold Path) which can defeat mara, but also one needs mindfulness in each and every moment, as the experience changes from rebirth to rebirth via one of our six senses!! 

Metta.