Preston’s Dog Food is a company owned by Preston and possibly Wendolene’s father before. In A Close Shave, Preston, a robotic dog designed to look after Wendolene (and may or may not have been built to shear sheep as well), begins turning them into dog food instead. Wendolene gets upset and orders Preston to stop the sheep thefts. Instead, he locks her and Shaun up with the intention of turning her (and the other sheep) into dog meat as well.
Later, in the factory, Wallace, Wendoline, Gromit and the flock are almost turned into dog food until Shaun comes to the rescue. In doing so, Preston ends up being minced himself as cogs, springs and pieces of metal filling up cans of the product. It’s unknown what happened to the company after Preston’s defeat and recreation, although near the end of the film, there’s an article in the newspaper mentioning that Preston’s Dog Food is under investigation for the sheep rustling after Gromit was cleared of the charges.