The current prevailing theory is that all mitochondria are descended from a bacterium that was swallowed by another prokaryote before the Eukaryotic cell even existed--so before animals, plants, fungi, algae, other protists, etc. split off from eachother. Because of that, it's believed that no living eukaryote developed mitochondria-like structures independently--they all come from the same original bacterium.
However, like everything relating to life, they have evolved quite a bit in different branches of the eukaryotic tree. In some eukaryotic microorganisms, [it's been highly modified and may not even perform the same function](https://en.wikipedia.org/wiki/Parabasalid
). The mitochondria of plants and animals, which are about as far apart as you can get on the eukaryotic tree of life, are fairly similar in broad strokes but have some differences like [the presence or absence of key enzymes](https://publishing.cdlib.org/ucpressebooks/view?brand=eschol&chunk.id=d0e6787&docId=ft796nb4n2&toc.depth=1
Within the animal kingdom, with a quick search I found that there are likely some biochemical differences between [mammal and reptile mitochondria](https://books.google.ca/books?id=hXY8DwAAQBAJ&pg=PA399&lpg=PA399&dq=mitochondrial+differences+within+mammals&source=bl&ots=022H6nqPGQ&sig=u478yvqTENqGBB2xNhiLatnOTB4&hl=en&sa=X&ved=0ahUKEwjJ76WuyLLZAhWMx4MKHQvVC-MQ6AEIXzAG#v=onepage&q=mitochondrial%20differences%20within%20mammals&f=false
), but I couldn't find anything stating that there are major differences between the mitochondria in mammals. It's very possible
Not an expert, just got interested in the subject, please feel free to add:
Additions: [[My thoughts on the subject, keep in mind my amateurism]]
The wealth of recent data from MA experiments across taxa provides a picture of the mutation spectrum that is far from evolutionarily constant. Mitochondrial genomes [[a mitochondrion has its own circular (not the X setup) DNA]] from yeast, worm, flies, and mouse experience qualitatively different mutational input [[So the DNA of the mitochondria can change]], yet maintain qualitatively similar nucleotide content [[Nucleotides are the building blocks of nucleic acids]] through a mutation-conversion-selection balance that remains to be explained. [[All the mitochondria can therefore create the same output with minor difference]]
While pervasive positive selection has recently been posited for the mtDNA, this theory remains controversial. The wealth of new MA data suggests that background selection must have strong effects on the evolution of a completely linked mitochondrial genome that experiences extensive purifying selection to remove mutations. Far from being a neutral molecule, the mitochondrial genome appears to have ample scope to be shaped by negative as well as positive selection.
[[So they have mostly the same role, got some only minor modifications but the extent of those modifications is unclear and controversial]]
Edit: for the last question, if technically possible, you could, this mitochondrion could do the minimum requirement. But IF a specific mitochondrion from a certain species got expert in reverting mutation (that's one of the theories) is inserted the giraffe (while at embryo stage) then the offspring of your mutated giraffe will mutate slower than the normal giraffe (because your mitochondria are cleaning every mutation you can have)