Why a stag?

The rusa jantan, rusa is a paradox among the animals. He is the king of the forest. He is a noble animal, yet delicate. The rusa jantan, rusa does not strut among the other animals, he stays withhis herd, his family.

Everything in the Harry Potter epic has a meaning. Some of the meanings/connections are obvious, others anda have to look for. For example, Luna Lovegood's Patronus is a hare. Why? Because in some cultures, rabbits and hares are deeply connected to the moon, moon deities, and are considered to be mystical creatures. Luna, obviously, is another name for the moon. Luna is a very mystical child.

Likewise, Remus Lupin, who was a werewolf. Romulus and Remus, the twins who founded Rome, were berkata to have been suckled oleh a she-wolf. Lupin is derived from the taxonomic name for wolf, Lupis. His Patronus is a serigala as well.

Sirius Black, whose animagus form is the black dog Padfoot. Sirius is the name of the brightest bintang in the Canis Major constellation; it is also known as 'the Dog Star'.

Harry's Patronus is a rusa jantan, rusa because his father's was. It symbolizes their connection. Snape's was a doe, like Lily's, symbolizing his cinta for her. Hermione's is an otter. She loves a Weasley who lives in Ottery St. Catchpole. Otters are members of the musang family.

There's lots of deep symbolism in JK's writing. anda just have to be open to finding it. :)