According to Harry Potter Wiki, Harry is NOT exactly a Horcrux, but rather the fragment of soul, latching onto to Harry for survival, is a side effect from Horcrux-making, as Voldermort had become "unstable". One may consider this a very unique phenomenon.
"Tamper with the deepest mysteries — the source of life, the essence
of self — only if prepared for consequences of the most extreme and
—The first of the Fundamental Laws of Magic To create a Horcrux is to
divide one's soul — the "essence of self" — and it is therefore in the
creation of a Horcrux that one falls prey to Adalbert Waffling's first
Fundamental Laws of Magic, which essentially states that tampering
with one's soul inevitably results in grave side effects. Creating
Horcruxes is considered one of the most dreadful acts possible.
A third side effect of Horcrux creation is that the master soul itself
becomes unstable (even with creating just one Horcrux). The precise
dangers of this spiritual destabilisation are not currently detailed
explicitly throughout the franchise, however, some can be gleaned
from the events in the books.
For example, the creation of Voldemort's sixth "Horcrux" — Harry
Potter — is known to be the direct result of this. When Voldemort
was hit by the back-fired Killing Curse at Potters' home in Godric's
Hollow, it caused Voldemort's soul to split, with one fragment
remaining in him and the other displaced part immediately seeking out
the only other living thing in the room and latching onto it — Harry
Potter. However, this parasitic fragment of Voldemort's soul that
attached to Harry was not actually a Horcrux, since it was not created
intentionally and the necessary parts of the Horcrux creation process
were not carried out.