Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Perseus.
mūlus, i, m. [perh. mu-; Gr. μυκάω; cf. μύκλος, an ass], a mule: muli pretio qui superant equos, Plaut. Aul. 3, 5, 21: rhedarii, Varr. R. R. 3, 17: clitellarius, Cic. Top. 8, 35: mulus vehiculo lunae habetur, quod tam ea sterilis sit quam mulus; vel quod, ut mulus non suo genere sed equis creatur, sic ea solis, non suo fulgore luceat, Paul. ex Fest. p. 148 Müll.: mulis celebrantur ludi in Circo Maximo Consualibus, quia id genus quadrupedum primum putatur coeptum currui vehiculoque adjungi, Paul. ex Fest. p. 148 ib.
As a term of abuse, you mule, you ass: mule, nihil sentis, Cat. 83, 3: muli Mariani, Marius’s mules, a nickname given to the soldiers of C. Marius, because they were compelled to carry their baggage on their backs like mules, Front. Strat. 4, 1, 7; Paul. ex Fest. s. v. muli, p. 149 Müll.; and s. v. aerumnulas, p. 24 ib.
Prov: mutuum muli scabunt, like the Engl. you claw me, and I’ll claw you, of those who flatter one another, Aus. Idyll. 12; hence: ridiculum est, cum te Cascam tua dicit amica, Fili Potoni, sesquisenex puerum. Dice illam pusam: sic fiet mutua muli, Poët. ap. Varr. L. L. 7, § 28 Müll.: mulum de asino pingere, a proverbial expression made use of when the original and the copy differ but little from each other, or when absurdities are represented by absurdities, or lies concealed with lies, Tert. adv. Val. 19 fin.