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.”