I would have interpreted the third horn as ears.

Two hypothesis on the six legs:

* an oddity, this things sometimes happen and might have been interpreted as good or bad omens, worth recording. The positioning (mid-flank rather than at the bottom, like the others) seems consistent with similar cases I have seen. Random internet result: http://www.huffingtonpost.com/2012/01/25/6-legged-lamb_n_123...

* maybe those are not extra legs of the top animal, but horns of the bottom animal (who also has 4 short legs), maybe recording when some ram or species of deer charged into a much larger cow? Though the horns would be out of proportion, and I'd expect them to be curved.

They are spears pointing to the vulnerable places to killing the uro? nape and two zones of the belly. Other pictures of the same series could be interpreted also as traps, and with a little of imagination, even pools of blood under both uros.

Perhaps it's some kind of ancestor of Sleipnir; https://en.wikipedia.org/wiki/Sleipnir.