This practise is called syncretism which means the blending, or fusion, of religious beliefs, myths and practices to form a new religious system. See the following links for more information on the similarities and differences between Greek and Roman mythology and religion:

Roman Mythology: Glossary of Terms
The Roman Mythology Glossary of Terms provides information and definitions regarding people and events who feature in Roman mythology. The subject of Roman Mythology is so vast that we found this the easiest way to provide an overview for our visitors. The following table provides an alphabetical list of terms closely related to Roman mythology, including the most famous Roman gods and goddesses.

Abundantia: Goddess of Abundance and Prosperity
Aesculapius: God of Health and Medicine
Ambrosia: Their food of the gods
Androcles: In Roman mythology, Androcles was a Roman slave who took a thorn from the paw of a lion. The lion later recognized the recaptured slave in the arena and spared his life.
Apollo: God of the Sun, poetry, music and oracles
Augur: An Augur was an ancient Roman priest, soothsayer and official whose main role was to interpret the will of the gods by studying omens, the flight of the birds, the behaviour of animals, interpreting dreams, natural phenomena or oracles referred to as "taking the auspices."
Auguries: There were five kinds of auguries which were used in Ancient Rome
Aruspices: Their role was to examine the animals offered in sacrifices on the altars of the gods, and by them to divine the success of any enterprise
Aurora: Goddess of the dawn

Bacchus: God of Wine
Bubona: Goddess of cattle

Candelifera: Goddess of childbirth
Carmenta: Goddess of Childbirth and Prophecy
Centaur: Was a mythical beast who was half horse with the head, torso and arms of a man
Cerberus: Huge and Savage dog with three heads which guarded the entrance to Hades.
Ceres: Goddess of Agriculture
Charites: The goddesses of gracefulness and the charms of beauty.
Charon was the ferryman who transported the dead across the river Styx to Hades
Chimaera: Was a mythological monster composed of the head of a lion, the body of a goat and a serpant for a tail
Clementia: Goddess of forgiveness and mercy
Cloacina: Goddess who presided over the sewers in Rome
Concordia: Goddess of agreement
Cosmogony: Story or myth which centered around the main Roman Gods and Goddesses
Cupid: God of Love
Cybele: Earth Mother
Cyclops: Mythical giant who had one eye in the middle of the forehead

Daemons (or genii): were an order of invisible beings
Deverra: Goddess of midwives and women in labor
Diana: Goddess of Hunting
Dii Consentes: Twelve major Gods in Roman Religion
Dis: In Roman mythology, Dis was the god of the underworld, also known as Orcus
Discordia: Goddess of Discord
Dryades: Nymphs of the woods and trees

Edesia: Goddess of food who presided over banquets
Elementals: Creatures or spirits of the elements
Epona: God of horses

Fabulinus: God of children
Fama: Goddess of fame and rumor
Fates: Fates was goddesses who decreed what would happen to both men and gods
Felicitas: Goddess of good luck and success
Fides: Goddess of loyalty
Flora: Goddess of Flowers and Spring
Fortuna: Goddess of Fortune
Fauna: Goddess of Animals

Genius: Was a spirit who was regarded as a protector through life
Graces: Graces, an alternative name for the Charites
Griffin: The griffin was a mythical monster with the body, tail, and hind legs of a lion, and the head, forelegs, and wings of an eagle.

Harpies: In Roman mythology the Harpies were creatures employed by the gods to carry out the punishment for serious crimes
Haruspicy: Augurs practised haruspicy which involved reading entrails of animal sacrifices
Hercules: God of strength
Hercules was also the name of the perfect athlete who was given twelve labours
Hespera: Goddess of dusk
Hesperides: Were daughters of Atlas and Hesperis
Hippona: Goddess of horses
Honos: God of military honours and chivalry

Invidia: Goddess of envy or jealousy
Iris: Goddess of the rainbow

Janus: God of Doors and beginnings and endings
Justitia: Goddess of justice
Juno: Goddess of Marriage
Juventas: Goddess of youth
Jupiter: King of the Gods and the God of the sky and rain (aka Jove)

Lares, or Lars: Roman household gods regarded as the deified spirits of mortals
Libertas: Goddess of freedom
Libitina: Goddess of death, corpses and funerals
Luna: Goddess of the moon

Mars: God of War
Mercury: Messenger of the Gods and of Commerce and Finance
Minerva: Goddess of Wisdom, the City, Education, Science and War
Mithras: God worshipped by Roman soldiers
Mount Olympus: The "home of the gods"
Muses: The nymphs of the springs
Muta: Goddess of silence

Necessitas: Goddess of destiny
Nectar: The drink of the gods
Nemesis: Goddess of revenge
Neptune: God of the Sea
Numa: the author of the religious institutions
Nymphs: Were higher beings than humans but not immortal like a god

Opis: Goddess of Fertility

Pax: Goddess of peace
Penates: The gods who were supposed to attend to the welfare and prosperity of the Roman family
Pietas: Goddess of duty
Pluto: God of the Underworld
Polytheism: Religion with many gods
Pomona: Goddess of fruit trees
Portunes: God of Keys
Proserpina: Goddess of Grain and Queen of the Dead
Psyche: In Roman mythology, Psyche was the personification of the passion of love boasted that she was more beautiful than Venus

Sancus: God of loyalty, honesty, and oaths
Saturn: God of Harvest and Agriculture
Satyrs: Were attendants to the god Bacchus
Sirens: Were daughters of a river-god and a Muse
Sol Invictus: sun God
Somnus: God of sleep
Sors: God of luck
Spes: Goddess of hope
Styx: Was the river in the underworld
Syncretism: The blending, or fusion, of religious beliefs and practices to form a new system

Tarpeia: Tarpeia was bribed by the Sabines to open one of the gates of Rome. The invaders were thrown from a cliff which was given the name of the Tarpeian rock
Tartarus: was the part of Hades where the wicked were punished
Tempestes: Goddess of storms
Titans: The Titans were the six sons and six daughters, the one-hundred-armed giants (Hecatonchires) and the one-eyed giants, the Cyclopes
Tranquillitas: Goddess of peace and tranquillity
Trivia: Goddess of magic
Turnus: Was the son of King Daunus and the nymph Venilia

Ultor: Ultor, meaning the Avenger, was a title of the Roman god Mars
Ulysses: Ulysses was the Roman name for Odysseus.

Venus: Goddess of Love and Beauty
Veritas: Goddess of virtue and truth
Vesta: Goddess of the Hearth and the Roman state
Victoria: Goddess of victory
Volturnus: God of Water
Voluptas: Goddess of pleasure
Vortumna: In Roman mythology Vortumna was goddess of oracles, the year and destiny 
Vulcan: God of Fire, the Forge and Blacksmiths