At the last moment Mollie, the foolish, PRETTY white mare who drew Mr. Jones's trap, came mincing daintily in, chewing at a lump of sugar. my question is: does Pretty an adjective or an adverb in this context?

The word "Pretty' as used in this context is an adjective.

Explanation:

Recall that an adjective is a word that modifies or describes a noun in a sentence.

It is used to describe the attributes of a noun. An example of an adjective is

John saw a beautiful lady yesterday.

Here, the adjective here is beautiful and it modifies the noun 'lady'

An adverb, however, shows the degree or extent of something by modifying a verb, adjective, determiner in a sentence.

An example is

The tortoise walked slowly

Here, the adverb here is slowly as it shows how the tortoise walked.

Therefore, the word "pretty'' in the given sentence is an adjective as it describes the noun 'mare'.