It is actually a wooden horse. It existed during the 12th century B.C, right at the time when Greece was declaring a way against the Trojan.
The legend is created as a result of the (real) Trojan War. It began when the prince of the Kingdom of Trojan, Prince Paris, fell in love with a girl named Helen. Unfortunately, Helen is the wife of Menelaus, the King of Sparta, a little kingdom under the power of Greece. In short, the beautiful Helen was successfully brought about to Trojan by Prince Paris. This made Agamemnon, the king of Greece, who is also Menelaus’s sibling, very furious. He suggested that it was an utter humiliation to the Kingdom of Greece, and he swore that until the last drop of his blood, he would pursue Prince Paris and conquer the Trojan empire, led by Achiles. A big war of Sparta + Greece against Trojan began.
Sparta and Greece deployed all of their troops to conquer and defeat Trojan, but it turned out that it was really difficult due to Trojan’s very strong castles and powerful troops. As a last attempt, Greece and Sparta surrounded Trojan with their troops for about 10 years. As the Greek troops almost lost faith, they discovered a tactic. They pretended to leave Trojan. They left giant horses, made of wood, just outside the city of Trojan, in which hid the Greek troops. Seeing the Greek troops as having surrendered, without suspicion the Trojans brought the wooden horses inside their city.
As the night came, the Greek troops came out of the wooden horses. They then opened the gate of the city, to allow the other troops to come inside. They finally burned and conquered Trojan, and successfully acquired Helen back.