All crests may extend to the base of the neck.
This is pretty self-explanatory.
Multi-pronged horns (antlers) that invariably start somewhere on the forehead or top of head. They must have branches of some kind.
Porcupine-like spines. They should be thin.
Horns with one starting point and one ending point. They can be twisted and curved however you like as long as you follow that rule. They should be placed symmetrically along the head and neck. You may choose the number of horns as long as they do not resemble sharp scales.
A crest that takes inspiration from aquatic life. Mainly fish, but dolphins and other creatures are acceptable. The crest will not extend past the middle of the shoulders.
A head of feathers that can resemble any real-world bird crest.
This can resemble any crest, as long as it is clearly made of metal, clay, or another artificial substance separate from the body material. It functions as a crown. It can merge with artificial snouts. More info here.
Takes inspiration from non-feathered crests of reptiles in general (not necessarily dinosaurs). Parasaurolophus, Carnotaurus, and Nyctosaurus are just a few examples of acceptable Saurian crests. It should have a noticeable real-world equivalent with similar shapes and proportions.