It's hard to narrow down just one animal that is bigger than we think. One great example would be the southern elephant seal, a species of seal found in the Antarctic and surrounding regions. Most people don't realize just how big they can get - the males can grow up to 5.8 m (19 ft) long and weigh up to 4,000 kg (8,800 lbs). That's absolutely huge! They are definitely an animal that we underestimate in terms of size.
Other animals that deserve a mention for being bigger than we think are the giant anteater, the reticulated python, the bison, and the saltwater crocodile. All of these animals are far larger than most people think - and they are also all very impressive in their own unique way!