Technically, there could be thousands of answers, as two animals can have the exact same hierarchy except for the species. There can not necessarily be two "closest related species".
There are, however, instances where animals of the same species have made different adaptations. Such as in one type of hummingbird living in South America, the female has a longer, more curved beak than that of the male because they feed on different flowers.
Humans and chimpanzees are the two species that are most closely related to each other.
Humans are most closely related genetically to chimpanzees.
Humans and chimpanzees are the two species that are most closely related in terms of genetic similarity.
Amphibians and avians are the most closely related species to reptiles.
Horses are related to one-hoofed mammals
The classification with the most similar species is the genus, as it groups species that are closely related and share common characteristics..species within the same genus are more closely related to each other than species in different genera.
The taxon that includes organisms that are most closely related is a species. Organisms within the same species share a high degree of genetic similarity and can interbreed to produce fertile offspring.
Humans are most closely related to chimpanzees and bonobos, sharing a common ancestor with them around 6-7 million years ago.
The okapi is most closely related to the giraffe. Family Giraffidae.
A genus is a low-level taxonomical rank that groups together closely-related species. These closely-related species have a common ancestor.
Members of a species are closely related and can mate to produce fertile offspring.
The kinds of evidence that indicates how closely species are related are evidence of DNA and protein structure.