As during menstruation, placental walls and the unfertilised ovum comes out through vagina , then what does during oestrous cycle comes out? Kindly answer sir/mam.

First I would like to correct you that in menstruation  which is found in primates, the prepared uterus lining  comes out not placental wall.

In oestrous cycle which is found in non-primates, no menstrual bleeding occurs. The cyclical changes in the ovaries occur due to the reproductive hormones in these mammals. But in absence of pregnancy, the uterus lining is not shed rather is reorganized for the next cycle, unlike primates mammals in which prepared uterus lining is shed in the form of menstrual bleeding in absence of pregnancy.