Final answer:
The poetry lines provided contain a caesura, which is a pause or break within a line of a poem used to emphasize word meanings.
Step-by-step explanation:
The lines of poetry "Beauty, thou wild fantastic ape, / Who dost in ev'ry country change thy shape!" utilize the poetic device known as a caesura. A caesura is a pause or break within a line of a poem, which is often used by poets to emphasize the meanings of words either directly before or after the pause. The appearance of the comma after 'ape' in the provided example indicates a slight pause in the middle of the line, exemplifying caesura.