How on earth should BOS come to mean cattle? Even if you think of Latin bos, bovis (which is cognate to English "cow") the expected form of a place name element should be derived from the oblique cases (like bovem) and look similar to boeuf or beef, not bos.
The Germanic word for cattle is fehu (Modern German: Vieh) and shows no similarity to bos at all.
Given the style of place names, BOS is most probably the relic of a given name (that maybe recoverable or not).