Down in the water his mother heard him,
Sitting in the sea-depths, beside her old father,
And she began to wail. (Iliad, 18, Lombardo)
Just as Helen, at every appearance she has, finds a way to talk about herself, Thetis is always mourning the eventual death of her son. She has long since left her mortal husband, and soon will lose her mortal son. But Thetis is best known in art, probably, in representations of her wedding to Peleus, that wonderful feast disrupted by Eris with her golden apple. Since her son is destined to be stronger than his father, Zeus has taken precautions by giving her a mortal husband, the fine, upstanding Peleus. It probably wasn't much of a marriage--he wins her by holding on as she shape shifts aggressively before finally giving in. I like to speculate about their marriage, but not here...
The image above is from the so-called Francois Vase, a volute krater by Kleitias, from around 370 BCE, which depicts the wedding procession at hand. It is characteristic that the Greeks would foreground the splendid physical qualities of their horses. Both horses and chariots were beastly expensive and difficult to maintain, but they were also both useful and attractive. The deliberate and forceful equine rhythms here are most pleasing to the eye.