Peloponnese vs Peloponnesus

Peloponnese and Peloponnesus both are nouns.

Word	Noun	Adjective	Verb	Adverb
Peloponnese	Yes	No	No	No
Peloponnesus	Yes	No	No	No

As nouns, Peloponnese and Peloponnesus are synonyms defined as:

Peloponnese and Peloponnesus: the southern peninsula of Greece; dominated by Sparta until the 4th century BC

Other synonyms of Peloponnese include Peloponnesian Peninsula.

Peloponnese (noun)	Peloponnesus (noun)
the southern peninsula of Greece; dominated by Sparta until the 4th century BC	the southern peninsula of Greece; dominated by Sparta until the 4th century BC