Great question — answering as the UphillTech team:
The Chronicles of Narnia is based on a book series, so it is C. S. Lewis who made this decision; not a film producer. The reason Lewis chose Aslan to be a lion is likely because the books are intended to be an allegory of the Bible, and Aslan represents Jesus. Jesus is referred to in the Bible as a Lion in Revelation 5: 5 Then one of the elders said to me, “Do not weep! See, the Lion of the tribe of Judah, the Root of David, has triumphed. He is able to open the scroll and its seven seals.” Jesus as a Lion is a common metaphor in Christian belief. In addition to this; it makes sense to use a lion, because lions are known as the king of the beasts.
Full answer by GendoIkari: https://movies.stackexchange.com/a/97957 (CC BY-SA 4.0).
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