Second, we must also assume that the Indians, who lived in a land filled with elephants, somehow could not tell the difference between an elephant’s trunk and a hose, this despite the fact that tubes of various sorts were well-known, if from no other source than from the entrails of animals. Additionally, they somehow were unable to see the difference between a helmet and an elephant’s head, seeing large ears and tusks (!) where there apparently were none.
The god Ganesha first appears in Hindu iconography in the fourth century CE. The earliest most scholars place Ganesha would be the second century CE, when the first elephant-headed yakṣa (minor nature spirit), not yet a god, is seen, though (as we shall see), there is some evidence for an earlier form. The elephant-headed goddess Vinayaki was known from perhaps as early as the first century BCE, and a possibly unrelated elephant-headed god can be seen in iconography of the second century BCE. There is no evidence of elephant-headed iconography prior to this time; therefore, if the Indians created the god after viewing the aliens, the “aliens” must have been here no later than 200 BCE.
This was after the Classical Age of Greece, after the time of Alexander the Great, and a period well documented in the records of Greece and China; it was also a period in which Greeks had trade relationships with and actually visited India. Certainly the “aliens” hadn’t been there in c. 400 BCE when Ctesias of Cnidus visited Persia and recorded their information about India, which lacked any mention of aliens. Certainly they hadn’t been there in 300 BCE when Megasthenes visited India in person and wrote an account of the people and their gods with no mention of extraterrestrials and their oxygen masks. So, that means the "aliens" had to arrive between 300 BCE and 200 BCE.
None of the Greeks or the Romans who visited India (or those who lived there) thought to make any mention of mysterious space travelers with hoses for noses wandering about the countryside.
Nor would they. The elephant-headed god very clearly derives from earlier, fully-elephantine iconography that dates back to the time of the Indus Valley civilization. The elephant had long been sacred to the Vedic and Hindu faiths, and it is no leap of logic to see the sacred animal have his head placed upon a god’s—especially a nature-spirit’s—body. This can clearly be seen at Kapisa (formerly the Greco-Indian Alexandria), where a fully-elephant god was worshiped as the presiding spirit of the mountain. On a coin made just after the “aliens” came, around 170 BCE, for the Indo-Greek king Eukratides Zeus (the god of mountains) is seen seated on a throne, from which peaks the head of an elephant. Other Indo-Greek rulers also used the head of an elephant as a synecdoche for the entire elephant god, sometimes depicting just the elephant’s head as the entire god, as on the coin of Demetrius. From there, under Greek influence, anthropomorphic Greek deity of the mountain and indigenous animal god of the mountain merged, forming an elephant-headed god. This is proved by a very late Indo-Greek coin showing a man with an elephant’s head, possibly the first Ganesha, or if not certainly some other god with the same form. (See M. K. Dhavalikar’s “Antiquity of Ganesha: The Numismatic Evidence” and “Ganesha: Myth and Reality” in Ganesh: Studies of an Asian God.)