Frogs, fenmen. The Mexicans are so called.
“When the Queen's Prize was won at Wimbledon, July 21st, 1885, by Sergeant Bulmer, 2nd Lincoln, his victory was hailed with `Well done, yellow-belly!' in allusion to his being a Lincon-shire man.” —Notes and Queries, August 22nd, 1885, p. 146.
“Ah, then, agin, it kin scarce be Mexikins neyther. It ur too fur noth for any o' them yellow-bellies.” —Captain Mayne Rcid: The War Trail, chap. 1xxi.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894