The Tibetan Buddhists have adopted much from Indian Buddhism, which again owes most of its content to Hinduism (Veda, Upaniṣads, Purāṇa, Itihāsa, Jyōtiṣa, āgama, Tantra, etc.) Tibetan astrology again is unoriginal for the most part. It derives its elements from various other cultures adding a Buddhist flavor to all its borrowings. Much of Tibetan astrology comes from pre-Buddhistic Bon which eventually got assimilated into Tibetan Buddhism and came to be regarded as a marginal school of Lamaism (Gyur Bon). Indian and Chinese systems of astrology also heavily influence what is today known as Tibetan Astrology. Jyōtiṣa is regarded as a vēdāṅga in Vedic culture and the influence of Indian Vedic astrology had reached Tibet even before its conversion into a Buddhist state. However, with the introduction of Kālacakra Tantra in Tibet, Indian astrology formally made its entrance as this Tantra faithfully reproduces older Hindu texts on Astrology.
Tibetans generally refuse to acknowledge the extremely evident influences of Indian and Chinese scriptures, be it on their Tantra, philosophy, or astrology. This includes not only several reputed Lamas I’ve studied under but also lay Western practitioners of Tibetan Buddhism. For example, Mañjuśrī, a Bodhisattva, is considered the originator of Tibetan astrology in its entirety. Tibetan tantric and astrological texts narrate a legend related to Mañjuśrī, which is clearly a hybrid-rehash of the older Indian tale of Kūrmāvatāra (the turtle incarnation of Viṣṇu) and the tale of Madhu-Kaiṭabha. The Tibetan version narrates the tale of Mañjuśrī, who, at the beginning of the present Kalpa when the future universe was still absent, caused a giant golden turtle to arise from his mind. This turtle emerged from the waters of the primordial ocean. He then pierced the flank of the turtle with a golden arrow causing it to sink into the ocean, spilling forth blood and excrement. From these arose the constituent elements of the universe. The created world thenceforth rested on the flat belly of the turtle. One can clearly notice here the Indian legends of Kūrmāvatāra where ādikūrma balances the earth on his back during samudra mathana. Also to note are the tales of Madhu-Kaiṭabha whose mortal remains contribute towards the creation and of ādiśēśa who supports the universe (here replaced by kūrma or the turtle).
The legend of Rāhu, one of the navagrahas (nine planets), is again adapted to Buddhist needs by Tibetan tantras. According to them, the universe was decimated by the poison hālāhala (they retain the same name as in the Hindu tale but modify its origin) administered by demons. In order to save humanity, the Buddhas churned the cosmic ocean with Mount Meru and generated amr̥ta (the legend of samudra mathana is copied viz-a-viz, replacing the devas and daityas with Buddhas). When this amr̥ta was collected, it was given into the keeping of Vajrapāṇi, a Bodhisattva. However, a demon named Rāhu was able to get hold of this amr̥ta and drank it all. He then fled into space and in his flight, threatened the sun and the moon with reprisals if they betrayed him. Vajrapāṇi, on the hot pursuit of Rāhu, was able to catch him finally, thanks to the information from the sun and the moon. He struck at Rāhu with his vajra separating his head from his body. Vajrapāṇi then had to drink Rāhu’s poisonous urine so that it would not be spread among humans and thus lost his golden luster and became black. Rāhu, who had become an immortal, transformed himself into a monster with nine heads and the body of a snake. He hunts down the sun and the moon periodically causing solar and lunar eclipses.
As one can see, the tale of Rāhu is completely adopted from Hindu mythology with purāṇic characters replaced by Buddhist figures. Chakrapāṇi Viṣṇu who severs Rāhu’s head and Nīlakaṇṭha Mahādēva who consumes hālāhala to protect the world, are both replaced by Vajrapāṇi, whose character is inspired by Indra, the bearer of vajrāyudha. The concept of Rāhu causing the eclipses is retained. Like most other things in Tibetan hagiography, Padmasambhava is also believed to have subjugated Rāhu and made him a Dharmapāla (Za Rāhula). However, this tale of subjugation of Rāhu at the hands of the mythical Padmasambhava fails to explain why the reformed Rāhu continues to attack the sun and the moon causing periodic eclipses.