Why is this the answer?

The answer is frog because this clever little riddle gives us two excellent clues that perfectly describe one of the most famous animals found near ponds and marshes. The frog is a champion of this puzzle! Let us look at the first clue: "I have four legs but no tail." When you think of a grown-up frog, its body is specially designed for jumping and swimming. A frog has two shorter front legs and two very powerful back legs that are much longer and stronger. These four legs allow a frog to leap an amazing distance through the air and kick through the water super fast. This is what makes it a four-legged animal. Now, remember what a baby frog is called. It is a tadpole! Tadpoles live completely in the water and have a long tail to help them swim, but as they grow up into a full adult frog, that tail slowly gets shorter and shorter until it completely disappears. So, the adult frog is left with only four legs and no tail, which is a perfect match for the first part of the riddle. The second clue is: "Usually I am heard only at night." The sound a frog makes is called a croak. If you live near a pond or a swamp, you know that the nights are often filled with a chorus of these strange, loud sounds. Frogs croak mostly at night for a few important reasons. First, the air after dark is cooler and has more moisture, which is much better for their skin because frogs are amphibians and need to stay wet to be healthy. Most importantly, many of the boy frogs, or males, use their croaking as a special song to call to the girl frogs, or females. They puff out their big, stretchy throats, which acts like a balloon, to make a very loud and clear sound so that other frogs can find them in the dark. They do this at night because it is safer from birds and other animals that might want to eat them, making their nighttime concert a secret way to find a friend.