Unlike some poems, "Turtle" does not have a mysterious title. On the face of it, the title is completely straightforward—a simple signpost to the topic of the poem. As you prepare to read the poem for the first time, you can confidently say, "Ah, yes. This poem must be about a turtle. No mystery there!"
By the end of the poem, however, you may have a slightly different take on the title. Yes, the poem is about a turtle, but in the process of learning about this turtle, you've probably learned something about human nature as well. So the poem isn't just about a turtle after all.
Maybe that's why Kay Ryan called her poem "Turtle" instead of "The Turtle." Including the word "the" would suggest that the poem was about a particular turtle. Without "the," the title seems more abstract and universal, encouraging symbolic interpretations (for more about this, hit up the "Symbols, Imagery, and Wordplay" and "Shout-Outs" sections).