Kinda like Tirek, except with far more finesse: The human doesn't just take magic; they take everything the creature had known. Information, spellcasting knowledge, anything and everything that could be stored in a living brain.

And a possible plot basis for this human: They get dropped in Equestria for any number of reasons, and initially run rampant with their ability to literally steal magic and knowledge from every living creature they encounter.

The twist? The human gets hit by a spell, one which separates them from their absorbed knowledge; from their home dimension, and the knowledge they took upon their arrival. Leaving them with only their personal history and identity intact.

But the human retains their ability to steal magic and knowledge. Stripped of their original wealth of stolen powers, and barely evading capture, the human goes into hiding, all while quietly stealing magic and knowledge from individuals that would not be missed (criminals, beasts, perhaps even some of the prisoners in Tartarus).

What happens from that point is entirely up to whomever uses this idea.

I'm looking forward to see his myself

5951354 As I read that, the only thing that was going trough my mind was that the stolen powers the human was separated from began to turn into a living creature rather than going back to their original owner.

5951354
You do know that anyone who experiences this will most likely end up with either double personality disorder or being in a catatonic state. . . right? I mean, the guy/gal will experience the entire life of the unicorn/magical being regarding how magic works and how to cast spells.

Nd to make matters worse, he/she has no control over it. Getting rid of this type of individuals is extremely easy: have either Celestia, Luna or Discord cast a magical spell on the human and the brain will have to handle thousands of years of magical experience from any of those three beings within a matter of seconds. Filtering that much information will deep fry the brain, because our brains aren't made to handle that much information in such little time.

What you could do instead of creating this magical leech of knowledge and arcane powers is something similar to this.

The human in question, when magic manages to do direct contact with the body, will grant access to the knowledge on how to cast that type of spell alone. While at the same time, the human absorbs the magic and stores it within, deciding what to do with it.

Either the human uses the stored magic to attack (either using the same spell or creating either an intense beam that will seriously hurt or a small condensed ball of magic that will expand so quickly that you can even state that it exploded). This will grant the human an increased immunity towards different types of magic (elemental, spiritual, arcane, holy, demonic etc) until the human reaches a point in the immunity in which the magic becomes nullified before it even does contact.

5951354
And a bit of food for thought

What would happen if a human like this manages to free the Umbra ponies from their prison and absorb all that deliciously toxic black magic from all of their bodies?

What would make an individual more powerful?
-absorbing the magic of what could be considered a god? (Celestia, Luna, Cadence, Twilight and/or Discord)
-absorbing the collective magic of a race that can bring the entire world down to its knees?

5951499
That could be a plot point to explore, should someone choose to write this. A semi-sentient blob of information and mana, that eventually becomes fully sentient, then starts becoming self-aware.

That much information and power becoming alive and intelligent, would be a scary thought indeed.

5951510

You do know that anyone who experiences this will most likely end up with either double personality disorder or being in a catatonic state. . . right? I mean, the guy/gal will experience the entire life of the unicorn/magical being regarding how magic works and how to cast spells.

You're putting too much real life logic into this: Magic doesn't exist in our world, but it did exist in the OP human's world of origin. So the mental physics are certain to be different.

Nd to make matters worse, he/she has no control over it. Getting rid of this type of individuals is extremely easy: have either Celestia, Luna or Discord cast a magical spell on the human and the brain will have to handle thousands of years of magical experience from any of those three beings within a matter of seconds. Filtering that much information will deep fry the brain, because our brains aren't made to handle that much information in such little time.

Same as above: Different world of origin, different mental structure to real life humans.

It's not supposed to be a super-realistic story idea. It's supposed to be an interesting one.

5952309
But if you make a character that simply can't be beat, then it becomes rather boring.

I mean, this mage/magus absorbs all magic when it does physical contact and gains the knowledge from the individual who casted that spell without overloading the brain because "magic"

If you are going to give a mage such indomitable power towards magic, then in order to counterbalance the absurdly OP innate talent the mage has, you must put a severe (even deadly) after effect of gaining the knowledge (the more elaborate and mana consuming the information of the spell being gained, the more taxing it becomes for the mage).

I mean, honestly, if this mage wants to conquer Canterlot, then no one in the MLP universe would be able to stop the mage for a simple reason.

Everything magical that does contact with the mage instantly looses its magic because it will be absorbed and he mage will know how to make everyspell the mage has ever done contact with.

For example

The mage touches Princess Celestia: can now control the celestial bodies and becomes immortal.
The mage touches Discord: can now control the fabric of reality and becomes immortal.
The mage touches Princess Twilight Sparkle: now has the same amount of mana/magicka reserves and all the spells Twilight knows and becomes immortal
The mage touches Starlight Glimmer: can now tear apart the space/time continuum in order to change the history of Equestria and beyond.
The mage touches King Sombra (or Sombras horn): can now use black/dark magic, transform into a shadow and summon/cast black crystals that prevent magic.
The mage touches Queen Chrysalis: can now gain the form and appearance of every single being (that has more or less the same amount of mass and dimensions as the mage).
The mage touches Princess Luna: can now access the dreams of everything that sleeps and becomes immortal.
The mage touches a cockatrice: can now petrify everything that stares him directly in the eyes.
The mage touches a pegasus: can now control, manipulate and stand on clouds.
The mage touches an earth pony: can now influence the growth of plant life.

This isn't an OP character, this is a character in God Mode.

5953904
How about this: The magic theft requires physical contact to be made with a victim. 10-30 seconds of unavoidable vulnerability, the duration dependent on the individual's total amount of magic being stolen.

Prolonged combat would still drain the human's stamina, and magic-theft does not substitute sleep, eating, or the call of nature for them. They are still, at heart, a creature of mortal flesh.