[1] The concept of an extremal black hole is theoretical and none have thus far been observed in nature.
It has been suggested by Sean Carroll that the entropy of an extremal black hole is equal to zero.
Carroll explains the lack of entropy by creating a separate dimension for the black hole to exist within.
[3] The Hawking radiation of extremal black holes is considered non-thermal (non-Planck distributed), with no associated temperature.
Nevertheless, recent work in a pair of preprints claims it contains an error and therefore extremal black hole are indeed possible.