The L&YR Class 21 is a class of small 0-4-0ST steam locomotive built by the Lancashire and Yorkshire Railway for shunting duties. They were nicknamed Pugs.

The class originates in the purchase of three saddle tank locomotives ordered from Vulcan Foundry in 1886. J. A. F. Aspinall then ordered more locomotives of a modified design, seventeen were ordered from Horwich Works in three batches; then Aspinall's successor Henry Hoy ordered another batch of 10; and finally, Hoy's successor George Hughes ordered 30 more in two batches.

The LMS gave the locomotives the power classification 0F. In total sixty were made between 1886 and 1910. They were built for use in sharply curved sidings for shunting duties. When the LMS was merged into British Railways on 1 January 1948, 23 'Pugs' remained in service; BR added 40,000 to their fleet numbers.

Two "Pugs" have survived into preservation, both through the Lancashire and Yorkshire Railway Trust.