I am going to say true.
"At the battle of Shiloh the Confederates left on the field a rawboned horse, very ugly and apparently good for nothing. As a joke, the officer who found this animal on the field, sent it with his compliments, to Colonel Lagow, one of my father's aides-de-camp, who always kept a very excellent mount and was a man of means. The other officers of the staff "jollied" the colonel about this gift. When my father saw him, he told the colonel that the animal was a thoroughbred and a valuable mount and that if he, Lagow, did not wish to keep the horse he would be glad to have him. Because of his appearance he was named "Kangaroo," and after a short period of rest and feeding and care he turned out to be a magnificent animal and was used by my father during the Vicksburg campaign." -Frederick Grant, son of General Grant