* It's likely a metaphor: The phrase seems to relate to a horse's tail being held high when running fast. This suggests a sense of urgency and speed.
* First recorded usage: The earliest known usage of the phrase dates back to the late 19th century, but it might have been used in a more colloquial manner before that.
* Possible influences: The phrase could have been influenced by other similar expressions, such as "beat it," "make tracks," or "get out of dodge."
In summary: While the exact origin remains unclear, the phrase "hightail it out of here" likely originated from a metaphorical association with a horse's high-tailing when running fast. It became popular in the late 19th century and has been used colloquially ever since.