Entomologist Piotr Naskrecki was taking a night stroll in a Guyana rainforest when he heard rustling in the underbrush. He switched on his flashlight expecting to see a small mammal, like a possum, but instead discovered a huge, puppy-size spider, reported CBSNews.

What he saw with the South American Goliath birdeater (Theraphosa blondi), cited by the Guinness World Records to be the world's largest spider. The spider's leg span can grow up to a foot -- the size of a child's forearm -- and develop a body the size of a large fist, Naskrecki, an entomologist and photographer at Harvard University's Museum of Comparative Zoology, told Live Science.

The Goliath birdeater can weigh more than 6 oz -- similar to that of young puppy, the scientist wrote on his blog.

Naskrecki also wrote that the birdeater's mammoth size is evident by its sounds.

"Its feet have hardened tips and claws that produce a very distinct, clicking sound, not unlike that of a horse's hooves hitting the ground, but not as loud," he wrote.

When Naskrecki encountered the intimidating spider in the rainforest, it rubbed its hind legs against its abdomen-propelling a cloud of hairs with tiny barbs that cause pain and itching if they enter the eyes or other mucous membranes.

But hairs with barbs aren't the Goliath spider's only defense: A pair of 2-inch-long fangs are ready to deliver a venomous bite. Luckily, the bite is not deadly to humans.

And the eight-legged beast has a third defense mechanism up its hairy sleeve. The hairs on the front of the spider's body have tiny hooks and barbs that make a hissing sound when they rub against each other, "sort of like pulling Velcro apart," Naskrecki said.

According to Naskrecki, the birdeater spider doesn't actually eat birds since it is nocturnal and hunts in a leaf litter on the ground. It has been known to puncture bird eggs and drink their contents. It eats small mammals, frogs and insects but prefers earthworms. The spider will attack anything it encounters, he said.

Naskrecki has been working in the South America tropics for several years and has come across the spider only three times in the last 10-15 years.

He captured the female specimen in Guyana and transported her to his lab for observation and study. She's now in a museum.

Many sources claim the giant huntsman spider is considerably larger than the birdeater with a wider leg span, but Naskrecki said the huntsman is more delicate than the beefy birdeater and comparing the two is "like comparing a giraffe to an elephant."