Here's why:
* Proper nouns refer to specific, unique people, places, or things (e.g., John, New York City, the Eiffel Tower). They are always capitalized.
* Common nouns refer to general categories of people, places, or things (e.g., boy, city, tower). They are not capitalized unless they are the first word in a sentence.
"Tarantula" is a common noun because it refers to a type of spider, not a specific, unique spider.