I think that if you really want Tarpan to have a more flavorful creature type,
it should be:
Creature - Uranium
   In the picture, the Tarpan is obviously undergoing fission. You can see it splitting apart!
Also, Tarpan is a 1/1. When it dies, you gain 1 life.
The difference in mass between 1 Tarpan, and 0 Tarpan, is 1.
That 1 point of mass became 1 point of energy (life), which is typical of a fission reaction (Loss of Mass = Energy).
Uranium is the usually the chief fuel for nuclear fission reactors, so I think that fits.
Of course, it could also be, "Creature - Heavy Metal", but that would just lead to a lot of confusion.
Also, if Tarpan was a Legend, it would be broken! You could activate its ability by playing another Tarpan!
There is a problem with your logic here... Uranium 238 splits to Thorium (if I remember correctly...lets go to wiki!!)
Nope. 235 splits to thorium. But I got the product right.
Regardless, it splits to thorium 231. That is a difference of
four. You are bad at maths.

In order to get one, you need a proton emission event. Since this happens relatively infrequently, we can only postulate that it is possible with something like tritium (which is an isotope of the element hydrogen, which has only one proton). This would destroy the element (like the Tarpan) and result in the 1 free change in energy (the released proton has a positive charge).
Therefore, I think that it
must be
Tarpan
G
Creature - Tritiated Horse
When Tarpan is put into a graveyard from play, you gain one life.
1/1
It's
obv not a NORMAL horse, so it retains it's special status. Plus, the laws of quantum chemistry are upheld. Voila!
Harkius