Answer:
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
<em>John saw a beautiful lady yesterday.</em>
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
<em>The tortoise walked slowly</em>
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'.