WARNING: Profanity and vulgarity (NOT suitable for any age)!
There is a myth that the Buddha and therefore Buddhists are and always were Hindus. Only Hindus say this, of course, in spite of the fact that the Buddha opposed the authority of the sacred Hindu scriptures (Vedas) and the traditions, sacrifices, and customs of the brahmins.
The Buddha, a wandering ascetic unencumbered by Temple politics, taught a path that did not depend on intermediaries for "salvation." He made known the liberating truth directly visible here and now. Practitioners penetrated it and directly experienced its benefits immediately. It is said of Buddhist teachings that they invite one to "come and see."
The benefits of doing so are not delayed, even if one does not immediately glimpse nirvana or become fully enlightened. Ordinary individuals listened to discourses, which were recited in standardized form then explained in detail, or asked questions. They gave attention to the Buddha's reply, committing it to memory, and reflecting on it. This led to insight, often right on the spot.
Yet, Hindus still claim the Buddha was merely an incarnation (avatar) of one of their gods. That is a wonderful compliment. And it is beautiful that, unlike traditional Jews and Rabbi Jesus, brahmins accept and even try to co-opt the Buddha, largely ignoring his teachings or reinterpreting them to teach the Vedas.
In an effort to discredit Buddhism and bring Indians back to Vedic Brahmanism (Hinduism), brahmins spread the idea that he was a Hindu god.
Hindus have done the same to Mahavira, the founder of Jainism, ignoring his teaching by holding him in high esteem for his severe asceticism and support of basic Brahminical beliefs (regarding the soul, karma, the afterlife, and the role of self-mortification in spiritual purification).
Why does Hinduism do this? It is because "Indus-ism" was not a religion but a collection of all things "Indian" lumped together by outsiders trying to make sense of the spiritual traditions of the Indus Valley Civilizations.
It became a "religion" (a dogma with a hierarchical structure that again put brahmin priests at the top) as Adi Shankara eventually bought into the need to organize the divergent schools under a single system. He standardized beliefs that became the hallmarks of Eastern Philosophy modern Hinduism.
Since the Buddha, perhaps the brightest luminary that ever came out of India, could not be excluded, he was subsumed within the central theistic Trinity (Brahma, Vishnu, Shiva).
Sadly, while it seems like a compliment, brahmins explain away his contrary views in all seriousness by saying that "God" in his incarnation as Vishnu was purposely trying to lead people away from the "eternal truth" (Sanatan Dharma) of the Vedas and Hinduism.
In Hinduism, the Buddha is regarded as the ninth incarnation of Vishnu following Ram and Krishna. That is obviously a real honor, even though Buddhism may not see it that way. Buddhism in fact does not believe in Gods and sees him only as an incarnation of a realized being. Nevertheless, various forms of the Buddha are worshipped in Buddhism in ways that are very similar to the worship of Hindu gods [particularly in Mahayana with its veneration of many buddhas and bodhisattvas].