Green’s Dictionary of Slang

prancer n.

also prauncer
[SE prancer, a mettlesome, spirited horse]
[mid-16C–mid-19C] (UK Und.)

1. a horse.

2. a highwayman.

3. a horse thief.

In compounds

prancer’s nab (n.) (also prancer’s nob, ...poll) [nab n.1 (1)/nob n.1 (1)/SE poll, head]

[late 17C–early 19C] (UK Und.) a horse’s head seal, when used for counterfeiting documents.

prancer’s poll (n.)

[late 17C–early 19C] (UK Und.) a horse’s head, usu. in ref. to the popular public house name, ‘The Nag’s Head’.