Since Jeffretariat is never seen it is not known what he looked like though he may have been a horse like his brother Secretariat.
It is not known what Jeffretariat's personality was like.
In The Shot, it is revealed Jeffretariat died in Vietnam, having been sent to war in his brother's place. The episode begins with a flashback to 1972. Secretariat is in the Oval Office begging President Nixon to not send him to Vietnam, as his celebrity status will cause a major backlash against protesters. Nixon tells him an arrangement could be made.
A while later, young BoJack is watching Secretariat, who has placed first in a race, on TV. Secretariat voices his support of the war as his brother, Jeffretariat, is fighting (as he was sent in Secretariat's place), and Nixon.
In present-day, when shooting in the Nixon Library, Kelsey tells BoJack he has been told his brother is dead and it's his fault, "But this moment is bigger than that. This is the moment that Secretariat stops running. Because this is the moment you realize something inside you is broken, and it can never be fixed."
BoJack looks sad and Kelsey says she got the shot, although BoJack didn't cry. She thanks BoJack for talking her into it, and says she's really glad they're making the movie together. BoJack says "Didn’t know I had it in me, did you?" Kelsey replies "No, I knew" as she leaves, to BoJack's surprise. He goes outside and lights up a cigarette, but after he exhales the smoke he breaks down crying.