Should 'lion' be capitalized?

No, the word 'lion' should not be capitalized. It's a standard noun describing a large wild cat species. However, if you wrote about the African lion, you will note that the word "African" is capitalized because it is common practice to capitalize the names of countries and places and words derived from those names.

It is interesting, by the way, that when I dictate articles as I am doing today using DragonDictate the software always capitalizes both lion and tiger. How about that? Very strange but it seems to be the default interpretation for this software which is, as mentioned, incorrect.