"Hullo, Jinty! Hullo, Pug! They're friends of mine. I don't know the others."
— Percy, The Eight Famous Engines
Jinty and Pug are two tank engines borrowed from the Mainland to run the North Western Railway while the engines visited England. Jinty ran Thomas' Branch Line while Thomas was away and also helped Thomas when the latter crashed into some buffers.
It is possible that Percy knows them from taking trains to the end of the line and telling the other engines stories, like at the beginning of Percy Takes the Plunge.