In the morning, it is a four-legged creature.
In the afternoon, it is a two-legged creature.
In the evening, it is a three-legged creature.
What is it?
Answer:
A human.
As a baby (morning), a human crawls on all fours like a four-legged creature.
As an adult (afternoon), a human walks upright on two legs like a two-legged creature.
As an elderly person (evening), a human may use a cane or walker, becoming a three-legged creature.