Antelope. (adult) the american version and not the normal "prong horn".
Im not saying its a Whitetail but an 18 inch spike is not a young'in. It has poor genes. My guess would be Elk or something else. We have a regulation here in some of the Everglades region management areas that a spike over 10 inches is legal game to weed them out.. The buck in my video is at least 2-1/2 likely 3-1/2 years old because I have been watching him. However 18 inches is a bit much and following the curve, its at least 19.18" spikes would be a hell of a whitetail young'in. Elk is likely (how far apart are the bases?). Maybe a mule deer?