In English, even though you're describing facts which may still be true, you still want to use the past tense in a narrative story like this. Why? Because you're describing the facts from the perspective of an agent in the past experiencing that fact in a past moment. It is true now, but it was also true then, and that's the part you want to emphasize. "One day, a fox saw a lion. The lion WAS big." "Before I saw the Eiffel Tower with my own eyes, I didn't know it WAS so tall." "I met a man yesterday. His name WAS Jack." You can only use the present tense when you are connecting the fact to some present or future situation. "One day, a fox saw a lion. The lion IS big, and the fox is worried that the lion is still out there." "I met a man yesterday. His name IS Jack. I think we are going to be great friends."

You have got a point, but in this case you should use the past tense because it is a tale (fictional writing) and it is told in the past. You would stick to the present tense when you are describing something that exists in the present or whose properties still exist in the presence. Example: "Donald Trump was sworn in on January 20. He is 70 years old." Note that in this case it would be wrong to write "He was 70 years old" even though the event occurred in the past because the subject or what you are saying about the subject is still true in the present. Making sense now?

The lion *was* big, he *had* a big mouth :)