There are many different mandalas, varying across time and text. Each however shares certain features. Mandala literally means circle, and Buddhist mandalas are typically in the form of a circle, or contain as a main feature an encircled area. At the centre of the mandala is the figure of a Buddha - who may be represented as a person, a symbol or a syllable. Surrounding the Buddha will be a number of other figures. If the Buddha in the centre of the mandala represents the cosmic principle of Awakening, then the other figures represent some aspect of Awakening.
Amongst scholars there is some debate about when esoteric Buddhism began. Some like Ronald Davidson argue that tantra proper is a feature of the 7th century. Others point to antecedents from earlier periods of both Buddhism and other religious traditions. For example the Golden Light Sutra is thought to belong to the 3rd or 4th century (it was first translated into Chinese in 414) and it contains what appears to be a mandala of four Buddhas. In the better known version of the sutra, that translated by Emmerick, the mandala appears in a Mahayana context without the usual esoteric features. There is another version of the Sutra, noted by Huntington , which makes it clear that the mandala is intended to feature in a meditation practice. This makes it seem more closely related to esoteric Buddhism, and adds weight to the "early tantra" theory. This kind visualisation is much older than we may think. There is, for example, a similar kind of practice in the Rig Vidhāna, a Brahminical text from fourth century BCE. 
We usually think of the five fold mandala - with a central Buddha surrounded by four other Buddhas - as being distinctively Buddhist. Early mandalas used a three fold symmetry. The five fold mandala emerges only in the 7th or 8th century in texts which the Tibetans call Yoga tantras. However I have discovered another five fold mandala in an unexpected place. Brhadaranyaka Upanisad III.9 describes the gods and their supports as occupying the centre and the four compass points: Agni occupies the centre, with Soma, Aditya, Yama, and Varuna occupying the north, east, south and west respectively. The list appears in a longish discussion between Yajnavalkya and Vidagdha Shakalya about how many gods there are. The Brhadaranyaka Upanisad is considered to be pre-Buddhist partly because the Buddha of the Pali Canon quotes from it, and satirises it in several places, e.g. the Tevijja Sutta, DN 13.
I think we have here a clear precedent for the type of mandala which becomes important in Buddhism many centuries later. This should come as no great surprise as we know, and I have tried to highlight this in my blog, that Indian religious traditions are quite free in adopting and adapting the practices and ideas of their 'competitors'.
What does this tell us about the origins of esoteric Buddhism? I think the best way to view esoteric Buddhism is that it was a grand synthesis of many religious traditions, with Mahayana Buddhism providing a framework, that occurred in the mid 6th century as a response to the socio-political and religious needs of the times. The fact that we find precedents may not be significant in determining the time of birth for esoteric Buddhism for this reason. To speak for instance of an Upanisadic mandala, or a Mahayana dharani as proto-tantric seems to create the wrong impression of the process. Sawn timber is not a proto-table for instance. Professor Ryuchi Abe makes a similar point in his discussion of the introduction of esoteric Buddhism to Japan.  Yes, many elements later incorporated into esoteric Buddhism existed before Kukai arrives back from China in 806. However Abe argues that there was no conception of "esoteric" Buddhism, no esoteric context in which those elements could exist. Buddhism in Japan up to 806 had been Mahayana Buddhism despite the widespread and frequent use of dharanis in ritual, and even translations of esoteric texts. Without the esoteric teachings such artefacts could only be interpreted through the lens of Mahayana Buddhism. This is further born out in several of Kukai's earlier works in which his main aim to establish the esoteric teachings as valid - Benkenmitsu nikyō ron, The difference Between Exoteric and Esoteric Buddhism, being one of the more important texts.
So Mandalas are quintessentially Buddhist, but have a history which pre-dates their use in esoteric Buddhism. Context is important in understanding any aspect of Buddhism.
- Huntington, John C. 1987. Note on a Chinese text demonstrating the earliness of tantra. JIABS 10 (2) p.88-98.
- Patton, Laurie. 2005. Bringing the gods to mind : mantra and ritual in early Indian sacrifice. Berkeley, University of California Press. p.30
- Abe, Ryuchi. 1999. The weaving of mantra : Kukai and the construction of esoteric Buddhist discourse. New York : Columbia University Press