Is there a succinct pronoun for a person who is a liar? You could make a phrase for a person who lies using mentior, or construct a sentence about the lie itself with mendacium, but what a single word for a person who is a Liar? It's hard to imagine that the Romans did not have a specific pronoun for this, but I have yet to find one.
According to L&S, the adjective mendax was used as a noun to mean liar:
I.given to lying, mendacious; subst., a liar.
I. Lit.: “mendacem esse adversus aliquem,” Plaut. Poen. 1, 2, 188: “cum mendaci homini, ne verum quidem dicenti, credere soleamus,” Cic. Div. 2, 71, 146: “Carthaginienses fraudulenti et mendaces,” id. Agr. 2, 35, 95: “aretalogus,” Juv. 15, 16.—As subst.: mendax , dācis, m., a liar.—Prov.: “mendacem memorem esse oportet,” a liar should have a good memory, Quint. 4, 2, 91.— Comp.: “Parthis mendacior,” Hor. Ep 2, 1, 112.—Sup.: “mendacissimus,” the greatest liar, Plaut. Rud. 3, 4, 48.—With gen.: “si hujus rei me mendacem esse inveneris,” Plaut. As. 5, 2, 4.—With dat.: “saepe fui mendax pro te mihi,” Ov. H. 2, 11.—With in and acc.: “in parentem,” Hor. C. 3, 11, 35; for which adversum, Plaut. Poen. 1, 2, 188.— With in and abl.: “in tenui farragine,” Pers. 5, 77.—
A good word for "liar," which can be used either as an adjective or as a noun alone, is mendax, -cis.
Here's Plautus in Truculentus:
D. Redin an non redis?
A. Vocat me quae in me potest plus quam potes.
D. Vno verbo
D. Mittin me intro?
A. Mendax es, abi. unum aiebas, tria iam dixti verba, atque ea mendacia.
My quick translation:
D: Are you coming back or not?
A: Someone is calling me who has more power over me than you.
D: One word!
D: Are you sending me in?
A: You're a liar, go away. You said "one," but you just said three [in English: five] words--and lying words too!
At quid interest inter periurum et mendacem?
What's the difference between a perjurer and a liar?