The diminutive Pug locomotive had its basis in the pre-grouping Lancashire and Yorkshire Railway, with the locomotives being built in batches by their designer John Aspinall, his successor Henry Hoy and his successor George Hughes.
The locomotives were designed as shunters for the northern region. Their small wheels and wheelbase made them ideal to negotiate the tight trackwork that is often found around docks and such. When the grouping act came into effect the locomotives were given the somewhat taunting power classification of 0F, such was their size and the specificity of their use.