The story of ‘Leda and the Swan’ is a Greek myth which is told in many variations.
According to one of them, Leda – a mortal queen, wife of the king of Sparta Tyndareus – was in the forest when Zeus spotted her and wanted to make love to her (or rape her). He transformed himself into a swan and landed next to her. They started playing and he seduced her. On the same night she also lay with her husband.
As a result, Leda ‘hatched’ two eggs, from one egg she bore the twins Castor and Pollux (Polydeuces), the fomer being the son of Tyndareus and mortal, and the other being the son of Zeus and therefore immortal. They are known as the Gemini (‘Twins’), the famous constellation, star sign. Through their love for each other, they both eventually became the immortal-mortals.
From the other egg she bore Clytemnestra, famous for becoming the wife of king Agamemnon, and Helen of Troy. No more is needed to say about the importance of these two women in the narrative of the dawn of the western civilization.
All these mythological- archetypical figures and life circumstances have fuelled the human imagination in the millenia and given such great material for the arts…. Here, I think, are two beautiful examples of this:
‘Leda with the Swan’ by Bartolomeo Ammaneti
Museo Nazionale del Bargello, Florence
Leda and the Swan
by William Butler YeatsA sudden blow: the great wings beating still
Above the staggering girl, her thighs caressed
By the dark webs, her nape caught in his bill,
He holds her helpless breast upon his breast.How can those terrified vague fingers push
The feathered glory from her loosening thighs?
And how can body, laid in that white rush,
But feel the strange heart beating where it lies?A shudder in the loins engenders there
The broken wall, the burning roof and tower
And Agamemnon dead.
Being so caught up,
So mastered by the brute blood of the air,
Did she put on his knowledge with his power
Before the indifferent beak could let her drop?
escape from a bankrupt state 
Is their any explanation, why Zeus always (?) disguised as an animal to have sex (willingly or by force) with the women? why did he not choose the human shape?
Hey Katrin! The explanation that I like is that these stories have incorporated older, more primitive ones, in their telling. In primitive societies some animals were worshiped as gods or had godly powers, and these characteristics, instead of dying out, were incorporated in the mythologies of later societies. I am sure there are other explanations too but this one makes sense to me as it argues that societies keep track of their past, and keep referring to those things that once carried powerful symbolic meanings to them.
[…] in a previous post I was taking about the inspirational drive that mythology has put into the arts, here is a great […]