Traits

A solid, smooth, regular neck with even fat deposits.

A leech's neck needs to be thick and elastic in order to swallow down large morsels of prey.  This particular model is the standard for the species, and all other neck traits have adapted from it.

A thick layer of padding has developed all the way around the leech's neck and used to store blood or fat. 

These leeches are big eaters but can go long times between meals which makes them a favorite pick as mounts for adventurers.  They tend to puff up even more after the leech has fed for a sometimes comical effect.

Fat and storage space have bunched up towards the top of the head to create a rolly, cushioned look similar to a thick hood. 

In the wild this gave a leech extra bulk to help it look intimidating, but owners of pet and mount leeches find it's a great trait for thick necked leeches who still need to be fitted with a collar or harness!

Built in a similar way to a camel, but as a large fat cushion on the back of the neck.

It gives the appearance of a hunch, even if the leech's neck itself is usually straight. Sometimes overfed leeches may find themselves weighed down by it.
It's not uncommon to see these leeches draped with bright fabrics and trinkets as the added space makes a great canvas for decoration.  It also makes a comfortable head rest for sleepy riders who need to go long distances.

Excess skin has developed and wrinkled at the base of the leech's throat to create this trait. 

Similar to the others, they fill out into a puffy flesh collar when the leech has consumed blood, often blending entirely into the rest of the neck and shoulders in cases when the leech has glutted itself.

The leech has developed a thick sac at the front of the neck that can be filled with air or fluid and used to make noise. 

Croak sacs are sometimes harvested and turned into satchels and bags, as the superstitious believe that the items stored inside them will never get lost.

Thick folds on either side of the neck that hang down into a loose hood shape. When the leech is full, they become dense, flaring out like a cobra.

Popular among collectors and as desert mounts, the cobra neck is often owned by an exquisitely patterned leech.  They are considered a symbol of prosperity in certain circles.
If the leech overfeeds, however, it looks extremely undignified; appearance-focused owners will do their utmost to ensure a balanced diet with no extra snacks.

Sometimes your cheeks just aren't big enough for all the blood you want to save for later.

A mythical trait where a leech grows a layer of fur over its neck to give it the appearance of having a fluffy collar. 

In most occasions the neck is the only place this fur will grow, but there are recorded incidents of furred leeches displaying this trait in the form of a noticeably thicker, extravagant collar or mane.

Sometimes pores in a leech's neck will develop a transparent skin over them, expanding and fleshing out into pockets for storage. 

Leeches featuring this trait will use them to show and display things that they're fond of collecting, whether it be physical objects, blood, or other foods. They tend to have much bulkier necks as well, both to have space for the neck cavities and strength to hold things stored inside.
It's extremely popular with luxuries and pets to have this combined with blush cheeks.

A rare mutation in which a leech sports a large frill of skin around its neck.

In the wild leeches use this frill to intimidate attackers, but they can also use it to express their emotions. A frill neck can just be plainly colored, but they often sport eye-like designs to add to the intimidation effect. 

Note: The frill can usually be raised and lowered at will, but can permanently be either or; and positioned anywhere on the neck itself.

[Potion item]: Magic Eclair

A neck that is extended up to one and a half times as long as the leech's own body. 

Leeches with this neck tend to be slender and notably more delicate. Because the length is so unstable, they often have to lie down when feeding to reduce risk of injury.
Necks that are twice as long as the body will almost always result in fatality due to blood being unable to reach the brain, and breeders frequently question how humane the trait is at all.