The French have a good slang term for these conveyances. They
call an omnibus a “Four Banal” (parish oven).
Of course, omnibus (for all) is the oblique case of omnes (all). Yet
Howitt, in his Visits to Remarkable Places (1840), says “Cabs
and cars and omnibi and stages” (p. 200). The plural of omnibus is
“omnibuses.”
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894