Tibet was commonly referred to in Sanskrit as Mahācīna, and Bhutan as Bhoṭānta. Mahācīna is not to be confused with Cīna which is the name for modern China.
Mahācīna is often mentioned in connection with the cult of Tārā, both in Hindu and Buddhist tantras:
Most important however in the alleged Buddhist connection is the story of Vasiṣṭha to be found in the Tantras. He is said to have gone to Mahācina (Tibet), which, according to popular belief, is half way to heaven. Mahādeva is said to be visible at the bottom of the Manasarova Lake near Kailāsa. Some of the Texts bearing on it have been collected in the Appendix to the edition of the Tārā Tantra which has been published by the Varendra Anusandhāna Samīti.
The Tārārahasya-vṛttikā of Śaṅkara Āgamācārya contains details on a ritual pertaining to Tārā called the Mahācīnakrama (“the Tibetan method”) (source).
And the following source connects both Mahācīna and Bhoṭa with various Hindu tantras.
P C Bagchi, on the basis of a sādhanā found in the Sādhanamālā, has tried to establish the identity of Mahācīnatārā with Ekajaṭā whose cult is said to have been recovered by Siddha Nāgārjuna from Tibet. The sādhanā of the goddess Ekajaṭā was discovered by him in the country of Bhoṭa. The description of Ekajaṭā is found in six different sādhanās and closely agrees with that of Mahācīnakramatārā. Corresponding to these goddesses, we find in the Hindu pantheon Tārā, Ugratārā, Ekajaṭā and Mahānīlasarasvatī. The dhyānas of these goddesses as found in the Hindu Tantras literally correspond to those found in the Buddhist sādhanās.