The scientific name of the Golden Toad is Bufo Incilius
periglenes. That means that it belongs to the genus Bufo, and the
subgenus Incilius (A genus is the smallest group above species, and
a subgenus is a division of a genus). Therefor, its closest living
relative would be a member of the genus Bufo, and the subgenus
Incilius.
There are 32 surviving members of the subgenus Incilius,
therefor any one of these could be the closest living relative of
the Golden Toad.