This little poem comes from the Florilegium Gottingense edited by Voigt and can also be found in the so-called Nevelet Aesop.
Quod non es, non esse velis; quod es, esse fatere:
Est male quod non est, qui negat esse quod est.
The vocabulary is keyed to the DCC Latin Vocabulary list. All the words in this poem are on that list:
fateor -ērī fassus sum: admit, confess
malus -a -um: evil
nego -āre: deny, refuse
qui, quae, quod: who, which, what; quis quid: who? what? which?
sum, esse, fuī: be, exist
volo velle voluī: wish, be willing
The image is an illustration of the Aesop's fable about the lion and the horse: