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!
Two snippets from suttas refering to Mara as an entity/position:
"But, venerable sir, in what way can a bhikkhu be called skilled in what is possible and what is impossible?"
"Here, Ananda, a bhikkhu undersands [...]: "It is possible that a man might occupy the position of Māra [...]"
The venerable Maha Moggallana saw him [Māra] standing there and said: "I see you there too, Evil One. Do not think 'He does not see me.' You are standing against the door bar, Evil One.
"It happened once, Evil One, that I was a Māra named Dusi [...]
A sutta where a disciple directly asks what is Māra (SN 23.1):
"Venerable sir, it is said, 'Māra, Māra.' In what way, venerable sir, might Māra be?"
"When there is form, Radha, there might be Māra, or the killer, or the one who is killed."
The above are from Nanamoli/Bodhi translations. Bhikkhu Bodhi also writes in a note of the Majjhima Nikaya:
The name means "the Corrupter" or "the Corrupted One". In the Buddhist conception of the universe, the position of Mara, like that of Maha Brahma, is a fixed one that is assumed by different individuals in accordance with their kamma.
There are multiple Maras 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!!