Learn Swedish: En tidig vår är ovanlig här. - An early spring is unusual here.

Even though it comes after the verb, the adjective ovanlig still has to agree with the subject of the sentence. Since en vår is an en-word, the adjective takes the en-form ovanlig instead of the neuter or adverbial form ovanligt.

I usually see the word early written as tidigt. Why is it tidig here?

Here, early describes the noun spring, making it an adjective. Because vår is an en-word, you use the en-form tidig. You use tidigt when it acts as an adverb describing an action, like in the phrase vi vaknade tidigt (we woke up early).

Is the o- in ovanlig the Swedish equivalent of the English prefix un-?

Yes, the prefix o- is used to make a word negative or opposite. Since vanlig means usual or common, adding o- creates ovanlig, meaning unusual.

Could I use hit instead of här in this sentence?

No, här is used for a fixed location (where something is), while hit implies motion towards a place (hither). Since a spring simply is unusual in this location, there is no motion, so you must use här.