Temple baptismal fonts are usually symbolized on the backs of Twelve oxen in remembrance of the Twelve Tribes of Israel.  Why is the Ox the symbol, instead of some other animal?

The ox was the banner, standard and ensign of Joseph, the birthright son, the father of Ephraim and Manasseh, which plays a key role in the restoration of the gospel in the latter-days.  Joseph had strength and power over his brethren because of his obedience to God.   In ancient Israel, the ox, “bull,” “wild bull,” (or “unicorn,” as it is rendered in the King James Version, Hebrew for wild ox) was a type or symbol of strength and power. (See Num. 23:22, n. 22a, below):

22. God brought them out of Egypt; he hath as it were the strength of a unicorn. (Footnote 22a Hebrew for 'a wild ox')

The Mesa Arizona Temple holds an annual Easter Pageant on the Temple grounds. Twice during the presentation, twelve men enter the stage.  Each holds a banner with a symbol of one of the twelve tribes, for instance, the Lion is symbol of Judah. The Ox, or bullock, is the symbol of Ephraim. (Mesa, Arizona web site)

In addition, Joseph Prado (1547-1595) (A Spanish Jesuit Biblical scholar) has said, "the head of a bullock (a castrated bull or steer) was depicted, because it was through the vision of the Kine or oxen that Joseph of Egypt had predicted and provided for, the famine in Egypt. (Pharaoh had a dream concerning famine and the seven Kine.  Kine are plural for cow.  The story is found in the entire chapter of Gen. 41).

16. And for the precious things of the earth and fulness thereof and for the good will of him that dwelt in the bush: let the blessing come upon the head of Joseph, and upon the top of the head of him that was separated from his brethren.  (Joseph was sold into slavery by his brothers, Genesis Chapter 37). The eldest, Rueben, lost his birthright because of sin; therefore the birthright fell upon Joseph).

17. His glory is like the firstling of his bullock, and his horns are like the horns of unicorns (or wild oxen): with them he shall push the people together (or gather) to the ends of the earth: and they are the ten thousands of Ephraim, and they are the thousands of Manasseh. (Deuteronomy 33:16-17, footnote 17b) (I Have a Question, March 1993, LDS.org)