Neck description is weird, anyway a brief tidbit of Cerulians. It's going to be a looooong description.
When first discovered Cerulians were refered to as "Giant frogdogs" due to their large size and apparently amphibious lifestyle. Although many characteristics determine it as a mammal. With it having hair/fur (length of hair and such defers from species to species) perform livebirth and provide their young with milk.

Cerulian are popular creatures in their world, Thiaru. They show amazing intellect as they seem capable of solving almost any kind of problem that comes their way. Despite being deeply admired for their intelligence, Cerulians are popular the world over for their capacity for complex emotion. There have been many records of Cerulians acting out human emotions. One famous story is of one a tropical behemoth father killing a band of poachers who had skinned its mate. This was determined as an act of revenge due to the Cerulian killing the select poachers involved in the deed. It is very easy to tell when a Cerulain is upset, angry, frightened or boastful.

Now with brief Descriptions of all of them.

Common Cerulian
When one is refering to a Cerulian, it's most often this fellow. The mane is found on both genders, although males are roughly more larger and bulkier than the females. This species is found all over the world from the cold seas of Kaldia to the red seas of Muhom. They often function in pods of 10 to 20 and can grow up to 7 feet long(head to tail) and 4 feet high. They project sound waves from their mouth in order to communicate. They can also use the fins on their heads to amplify the soundwaves thus making them harmful to others, this is the common cerulians primary weapon. These amplified sound waves can cause serious internal injury, any prey unlucky enough to get caught is generally doomed to be eaten.

Mime Cerulians
Many think that the mime cerulains get their name for their white skin and black markings although, that is not the only reason. Mime cerulians rarely work in groups and are roughly 5 ft long and 3 ft high. As you can see the mime cerulian is significantly smaller than its common cousin and weaker as it possesses no amplifier fins. Although this cerulian has a very unique trait. During the early stages of its life it appears the way it is presented on this sheet. A mime cerulian is know to stick to one animal in its diet or eat one more often than any other throughout the first part of its life. As grows into adulthood, it obtains an eerie resemblance to the animal it has eaten for most of its life. The mime will grow fur if it is necessary for this..'metamorphasis' After this change its diet expands greatly. This species is found everywhere except Lamnia.

Tropical Behemoth
The largest member of the group, the TB can grow up to 10 ft long(this measurement does not include the curled tail which can be as long as 7 ft when uncurled.) and can also be 5 feet tall from the head to the toes. Only the males grow fur, thet pattern of said fur resembles that of a male lion found on our planet. These collosal beasts are found primarily in warm subtropical areas such as Hayashi and some northern parts of Muhom. They can use powerful shockwaves much like the Common Cerulian, and use their long curled tails for whipping prey and other Cerulians into submission. Tropical behemoths are infamous for their aggressive behavior. They function in extremely small and strict pod units that consist primarily of a father his harem(somethimes he just has one mate) and the pups. Each of these pods has a strict area of territory which is monitored and protected by the alpha male . Fights between Tropical behemoths are notorious bloody.

Crimson Ghoul
This animal spends the majority of its life in the abyssmal depths in the Lamnian ocean. It is the smallest member, being 3 ft long and 2 ft tall. People believed for a long time that this animal was just another mysterious fish of the deep that happened to look like the nightmarish version of a Cerulian. Although close examination of the corpses of this species showed that this animal had many genetic similarities to Cerulians. Such as similar muscle structure(despite it being hideously emacitated), similar bone structure and many of the same exact organs...INCLUDING lungs. Although these lungs are usually horribly shriveled in the autopsies. The crimson ghoul seems to rely heavily on some highly complicated form of respiration through the skin covered in tiny pours which extracts oxygen from the ocean and put it directly into the blood stream. The lungs in this creature maintain its buoyancy underwater. The only time this Cerulian ever uses its lungs in the same way as its cousin is at the very end of its life. Where it will actually swim to the surface(either alone or in groups) to sing a haunting hymn, they die shortly afterwards(swansong). People that have witnessed this behavior first-hand argue that it is the most beautiful sound in the world. Not much else is known about this species.

Thiaru and Cerulians are things I made up.