My Military History professor, and an article in American Rifleman (don't remember which issue, it was some time ago) both stated that Custer had three M1866 Gatling guns available to him, and although they were cumbersome to the point of being something that was utilized more like defensive artillery, would still have been a battlefield asset. (Had he not jumped to conclusions and prematurely initiated the attack.) One he chose not to utilize because he wrongly, as history proved, thought maneuver would be more an advantage than it was. In actuality, accepting the Gatlings would have forced him to not impulsively attack the camp at LBH and try to hog the glory for himself. (A known character defect of Custer's that had on occasion worked for him against the Confederates)
Yes, a lack of good horses, and having poor horses was a major factor, but more so because of Custer's 'blitzkreig' attempt at taking hostages instead of waiting for his own baggage train under McDougall, much less the reinforcements coming from Terry and Gibbon. Custer did not believe his own scouts, both internal and attached, and did not see what his scouts saw when visiting them. This, coupled with assuming (you know what happens when you assume..) that outbound Indians from the village were scouts and had reported his position (which at the time was very secure from attack) to the village, leading him to his rash, and ultimately, unsuccessful attempt at taking hostages. Reno and Benteen, not being as forward as Custer, were able to withdraw in good order to a hill where they were able to fend off repeated attacks until Terry relieved them on the 27th. The Indians withdrew, Terry's men treated Reno and Benteen's men, including Benteen himself. they then withdrew to the steamboat Far West, which carried the wounded back to Ft. Lincoln.
In the end, it was Custer's hubris that doomed himself and his men at the Little Big Horn.