Oestrus cycle is a reproductive cycle that occurs in most mammalian species except for humans. It is characterized by a period of sexual receptivity or "heat" during which the female is most likely to mate and conceive.

The animal in which oestrus cycle is absent is a monkey. Let's discuss the oestrus cycle in other animals for better understanding:

Sheep:
- Sheep have a 17 to 19-day oestrus cycle.
- The cycle can be divided into four stages: proestrus, oestrus, metestrus, and diestrus.
- During the oestrus stage, the ewe is receptive to the ram and ovulation occurs.

Tiger:
- Tigers have a 3 to 4 week oestrus cycle.
- During the oestrus stage, the female is receptive to the male and ovulation occurs.
- If mating occurs, the female may become pregnant.

Rats:
- Rats have a 4 to 5-day oestrus cycle.
- During the oestrus stage, the female is receptive to the male and ovulation occurs.
- If mating occurs, the female may become pregnant.

Monkey:
- Unlike other animals, monkeys do not have a distinct oestrus cycle.
- Instead, they have a menstrual cycle similar to humans.
- The menstrual cycle is characterized by a monthly shedding of the uterine lining and a corresponding drop in estrogen levels.

In conclusion, while sheep, tigers, and rats have distinct oestrus cycles, monkeys do not. Instead, they have a menstrual cycle similar to humans.