a wild beast: a dangerous, untamed animal, and by extension the savage power Revelation calls the Beast
The word names a wild animal, the kind that is dangerous and cannot be safely handled: a predator, a venomous snake, a creature of field and desert. It is not the ordinary word for an animal you keep or herd. It carries the sense of something fierce and untamed, the opposite of a tame or domestic creature.
Because of that fierce, predatory feel, the same word becomes the great symbol of the book of Revelation. There the Beast is not a literal animal but a brutal power that wars against God and his people. The image works precisely because the everyday word already means a savage creature you cannot tame or trust. Calling the persecuting power a beast says, in one word, that it is predatory, monstrous, and beyond reasoning with.
The two uses are one picture seen at two levels: the actual wild animal, and the human or spiritual force that behaves like one. Most of the appearances of the word in the New Testament are the symbolic Beast of Revelation; the rest are ordinary wild animals.