Poseidon the Earth Shaker:
In Greek mythology and legend, Poseidon is the god of the sea. However, his domain includes some aspects of the land as well, and in fact he is known as “earth-shaker” in many stories, because of his penchant for causing earthquakes. Poseidon was responsible, according to Greek legend, for the collapse of the Minoan civilization on the island of Crete, which was all but destroyed by a giant quake and tsunami.
The Battle for Athens:
One of the twelve gods of Olympus, Poseidon is a son of Cronus and Rhea, and brother of Zeus. He battled Athena for control of the city which would later become known as Athens, in honor of the victor of that dispute. Despite Athena’s role as the patron goddess of Athens, Poseidon played an important role in the city’s daily life, sending a giant flood to punish the Athenians for not backing him in the fight.
Poseidon in Classical Mythology:
Poseidon was a very important deity in many Greek cities, including but not limited to Athens. He was honored on a regular basis with offerings and sacrifices, particularly by sailors and others who made their livings from the sea - fishermen, and those who lived along the coastlines wanted to keep Poseidon appeased so he wouldn’t cause a devastating earthquake or flood.
Sometimes horses were sacrificed to Poseidon - the sound of his roaring waves were often associated with horses’ hooves - but Homer describes in the Odyssey the use of several other animals to honor this deity:
Take an oar, until one day you come where men have lived with meat unsalted, never known the sea... and make a fair sacrifice to Lord Poseidon: a ram, a bull, a great buck boar.
Pausanias described the city of Athens and its Hill of Horses, and makes a reference to both Athena and Poseidon as being connected to the horse.
There is also pointed out a place [not far from Athens] called the Hill of Horses, the first point in Attika, they say, that Oidipous reached--this account too differs from that given by Homer, but it is nevertheless current tradition--and an altar to Poseidon Hippios (Horse God), and to Athena Hippia (Horse Goddess), and a chapel to the heroes Peirithous and Theseus, Oidipous and Adrastos.
Poseidon also makes an appearance in stories of the Trojan War - he and Apollo were sent to build walls around the city of Troy, but the King of Troy refused to pay the reward he had promised them. In the Iliad, Homer describes Poseidon’s rage, in which he explains to Apollo why he is angry:
I walled the city massively in well-cut stone, to make the place impregnable. You herded cattle, slow and dark amid the upland vales of Ida's wooded ridges. When the Seasons happily brought to an end our term of hire, barbaric Laomedon kept all wages from us, and forced us out, with vile threats.
As vengeance, Poseidon sent a giant sea monster to attack Troy, but it was killed by Heracles.
Poseidon is often depicted as a mature, muscular and bearded man - in fact, he looks remarkably like his brother Zeus in appearance. He typically is shown holding his powerful trident, and is sometimes accompanied by dolphins.
Like many ancient gods, Poseidon got around quite a bit. He fathered a number of children, including Theseus, who slew the Minotaur on the Isle of Crete. Poseidon also impregnated Demeter after she had rejected him. In hopes of hiding from him, Demeter turned herself into a mare and joined a herd of horses - however, Poseidon was smart enough to figure this out and turned himself into a stallion. The result of this not-entirely-consensual union was the horse-child Arion, who could speak in the human tongue.
Today, ancient temples to Poseidon still exist in many cities around Greece, although the best-known may well be the sanctuary of Poseidon at Sounion in Attica.