This well-modelled image of Nandi, the vahana or celestial vehicle of Shiva (known as Phra Isuan in Thailand), is from the Rattanakosin period of Bangkok. The image is shown standing on a rectangular platform, with ample neck creases, a pair of elegant horns, and a long tail. There are various areas of floral-like decoration cast in relief including a decorative blanket over the back that has been finished with gold leaf in typical Rattanakosin style. The top of the platform is lacquered red.
The rendering is unusual because in Thailand typically such Nandi images are cast with Shiva, with multiple arms, standing atop Nandi, but that is not the case here, nor are there any signs as to how a Shiva image might have been attached. Instead, clearly, this is a standalone image.
Thai Buddhism is unusual in its direct incorporation of Hindu deities into its pantheon of deities. Hinduism was the main religion of the region of what is now Thailand before the arrival of Buddhism and many vestiges of Hinduism survive and were incorporated into Thai Buddhism. Chakri Dynasty Thai kings, though Buddhist like most Thais (indeed, Thailand is the largest Buddhist country in the world), have Hindu Brahmin priests officiate at their coronations, and the kings themselves are seen as manifestations of Vishnu. The emblem of the Chakri Dynasty is Garuda, the vahana of Vishnu.
The image here is very decorative. The bronze has a wonderful colour, and the condition is excellent.