Varro, On the Latin Language, 148
“In the forum one finds the Lacus Curtius (“Pool of Curtius”), which opinion holds was named for Curtius. But the story about why has three forms: Procilus does not report the same thing which Piso does and Cornelius doesn’t follow it either. Proclius reports that in this place the earth opened wide and this fact was referred by senatorial decree to the haruspices: they responded that the gods of the dead asked for completion of a vow that had been forgotten: a promise to send down the bravest citizen. At that time, a certain Curtius, a brave man, armed, climbed atop his horse, and, after he turned from the temple of Concord, threw himself into the hole with his horse. When that deed was done, the place close and entombed his body divinely: it created a monument to his family.
In his Annales, Piso writes that during the Sabine war that occurred between Romulus and Tatius, a most stout Sabine named Mettius Curtius, at the moment when Romuus brought his men on a charge from higher ground, escaped into a marshy spot which was then what the Forum was because the sewers were built and then retreated to his own men on the Capitoline. Well, Piso records this is how the place got its name.
Cornelius and Lutatius write instead that the place was struck by lightning and as a result was fenced in by a senatorial decree. This was done under the leadership of a Consul named Curtius who was a colleague of Marcus Genucius. For this reason, it was named Lacus Curtius.”
Cornelius et Lutatius scribunt eum locum esse fulguritum et ex S. C. septum esse: id quod factum esset a Curtio consule, cui M. Genucius fuit collega, Curtium appellatum.
In Foro Lacum Curtium a Curtio dictum constat, et de eo triceps historia: nam et Procilius non idem prodidit quod Piso, nec quod is Cornelius secutus. A Procilio relatum in eo loco dehisse terram et id ex S.C. ad haruspices relatum esse; responsum deum Manium postilionem postulare, id est civem fortissimum eo demitti. Tum quendam Curtium virum fortem armatum ascendisse in equum et a Concordia versum cum eo praecipitatum; eo facto locum coisse atque eius corpus divinitus humasse ac reliquisse genti suae monumentum.
Piso in Annalibus scribit Sabino bello, quod fuit Romulo et Tatio, virum fortissimum Mettium Curtium Sabinum, cum Romulus cum suis ex superiore parte impressionem fecisset, in locum palustrem, qui tum fuit in Foro antequam cloacae sunt factae, secessisse atque ad suos in Capitolium recepisse; ab eo lacum Curtium invenisse nomen.