This question came about from reading the comments of this (very unclear) question, which the author did not properly clarify.
Vegans are often recommended to take vitamin B12 supplements, as the vitamin is not present in sufficient quantities within vegan food. This implies that plants do not produce (or need) vitamin B12 in their metabolic processes.
However, herbivorous animals do not require any vitamin B12 supplementation, yet contain significant amounts of vitamin B12 in their food products.
What, then, is so different about the digestive tracts of humans and other herbivores that prevent humans from obtaining sufficient vitamin B12 from bacterial sources? Can humans be supplemented with gut bacteria from herbivores, allowing them to obtain sufficient vitamin B12 while on a vegan diet without requiring supplementation?