When describing something as a "big beast," there are many synonyms that can be used to add variety to your language and make your writing more interesting. One such synonym is "giant creature," which is both descriptive and evocative. Another option is "monstrous animal," which conveys a sense of danger or menace. "Colossal beast" is another possibility, highlighting the sheer size of the creature in question. "Massive predator" brings to mind the idea of a fearsome hunter, while "enormous brute" is a more aggressive term that suggests a creature to be feared and avoided. Whatever synonym you choose, be sure to take into account the tone and context of your writing, as well as the image you want to convey to your readers.