Here I use Hero Forge to depict the ongoing transformation of Atalanta and Hippomenes into lions.

Mythology (as I know it)
Atalanta (also Atalante) was born the daughter of a king.
But because her father only wanted sons, he abandoned her in the forest, as he would otherwise incur the wrath of the gods if he killed his child himself.
But Atalanta was taken in and raised by a bear.
The wild girl was found by hunters and taken in by them.
Atalanta became "civilized" and, as she grew into a woman, showed special talent in archery, hunting, and racing.
She even hunted alongside the huntresses of the goddess Artemis, but without taking the oath of chastity and celibacy.
Atalanta became a member of Jason and the Argonauts, who set out to retrieve the Golden Fleece from Colchis. [However, she is only a minor character in the Argonauts' legend.]
She played a larger role, however, in the hunt for the Calydonian Boar.
Artemis sent the Calydonian Boar to punish King Oeneus of Calydon for forgetting her during the first fruits sacrifice. Offended that he had ignored her during the harvest sacrifices, she sent the giant wild boar to ravage the land and terrorize the inhabitants.
During the hunt, Atalanta fell in love with Meleager, the prince of Calydon (although it is also said that he was a son of the war god Ares).
Meleager and Atalanta managed to kill the boar.
Meleager gave Atalanta the best part of the slain boar, which angered his uncles (who believed that a woman should not receive the best part).
A fight ensued, in which Meleager killed his uncles.
When his mother learned of her brothers' deaths and that her own son had done the same, she threw a log, on which her son's life depended, into the fire.
She killed herself, certain that she had condemned her son to death.
Meleager died in Atalanta's arms. (It is said that she conceived and bore a son by Meleager, whom she named Parthenopaeus, and who, as an adult, fell in the battle of the Seven against Thebes.)
Atalanta was taken in by her father, who had her abandoned, and recognized as a princess (surely because he had heard of her fame).
But an old, clairvoyant woman prophesied to Atalanta that if she married, disaster would befall her.
According to some versions, Atalanta's father decreed that anyone who sought Atalanta's hand should try to defeat her in a footrace. The winner would marry Atalanta, and the loser would pay with his life.
Many men, captivated by her beauty and the prospect of becoming king, were willing to compete against Atalanta. But Atalanta defeated one suitor after another, and they were sentenced to death.
Then Hippomenes appeared (some said he was a son of the sea god Poseidon) and fell in love with Atalanta.
Like all other suitors before him, he had to try to defeat her in a race.
He went to the temple of the goddess of love, Aphrodite, and asked her for help.
The goddess appeared and gave him three golden apples (it is said they came from the Garden of the Hesperides).
After Aphrodite explained to Hippomenes the role the apples would play in the race, she made him swear to pay her appropriate tribute if he defeated and married Atalanta.
The moment arrived, and Hippomenes and Atalanta were ready for the race.
The race began, and as Atalanta tried to catch up with Hippomenes, he dropped one of the golden apples.
Atalanta stopped to pick up the apple, which helped Hippomenes overtake her.
Hippomenes dropped the second and then the third apple, which distracted Atalanta and allowed him to win the race. Atalanta and Hippomenes married, but they forgot to pay the promised tribute to Aphrodite.
Aphrodite became very angry about the forgotten promise of tribute, since she had made the marriage possible, and swore revenge on the two.
When Atalanta and Hippomenes went hunting, Aphrodite ambushed them and cast a curse upon them.
The curse awoke uncontrollable lust in the couple, and they sought a place to satisfy their desires.
Fatally, they entered a temple and had sexual intercourse.
It was a temple of Zeus, the ruler of Olympus. Zeus saw the couple and was deeply enraged by this form of desecration.
As punishment, he transformed them into lions, and they were forced to live in the wild from then on.
It is said that they could not produce offspring, as the Greeks believed that lions did not mate with other lions, but only with leopards, which made their punishment even more severe.
Others say that the lions Atalanta and Hippomenes spawned a new generation of lions with intelligence similar to humans.
Another version says that the two had sexual intercourse in the temple of the Titaness Rhea (mother of Zeus, Poseidon, Hades, Hestia, Hera, and Demeter). As punishment, Rhea not only transformed Atalanta and Hippomenes into lions but also condemned them to pull her chariot.
Another version says that it happened in the temple of the goddess Cybele, and that she was the one who condemned Atalanta and Hippomenes to life as lions.
Another version says that Atalanta and Hippomenes were transformed into stone lions.