San Pedro has the strongest archaeological case of any sacred plant in South America, which is rare, because ritual plant use is exactly the kind of practice that normally leaves nothing behind.
At Chavin de Huantar in the Peruvian highlands, a ceremonial centre in use from roughly the first millennium BC, a stone relief shows a fanged anthropomorphic figure holding a columnar cactus, and the cactus is rendered with enough anatomical accuracy that the identification is not seriously contested. Cactus imagery recurs in Cupisnique and later Moche and Nazca material, and dried cactus has been recovered from dry coastal sites. The site itself is built with galleries, drains and acoustic features that a number of archaeologists read as designed to disorient, which fits.
The name is the interesting distortion. San Pedro, Saint Peter, is a colonial-era Christian overlay: the saint holds the keys to heaven, and the cactus is said to open the gate. It is a piece of syncretic wit, and it did its job, letting a suppressed practice continue under a name no priest could object to. The older Quechua name wachuma has been returning to use, and the choice between the two names is now itself a statement about whose tradition it is.
A caution: several Trichocereus species are involved and the taxonomy has been repeatedly revised, so older sources and modern ones do not always mean the same plant.