Right off the bat, I want to let you know that area here is a relative term. These are not necessarily cryptids that are exclusive to Boston, but ones that are in Massachusetts. With that disclaimer out of the way, here are three creatures to keep an eye out for.

Bigfoot

Bigfoot, sasquatch, whatever you want to call him, he can be found up in the woods of Massachusetts. The closest a bigfoot has been spotted near Boston was in the Blue Hills reservation SW of Quincy. As bigfoot is kind of an entry level cryptid, I won't spend too much time on him. A bigfoot is a big ape man that likes to keep to himself.

Night Walkers

Night Walkers are humanoid creatures that have hooved feet, claws, and have the face of a dog. They have a foul smell and are said to be covered in mold. Gross! They seem to live in underground spaces and only come out at night. Old folklore says that they can be approached and bargained with, but you will need to pay a hefty price for what you want. I don't buy that part, though.

The Boston Harbor Frogman

The Boston Harbor Frogman is an amphibious creature that has been occasionally spotted in, you guessed it, Boston Harbor. Unlike other aquatic cryptids, the Frogman is not a serpent, but has a humanoid form with appendages, hence the "man" part of the name. He can either hold his breath for an untold amount of time or is able to outright breath under water.

There have also been claims of other Frogmen inhabiting the sea near other coastal towns in Massachusetts, especially further up north.