The Beached Things (BTs) are entities that come into existence when the deceased soul of a living being fails to cross through the Beach into the world of the dead, consequently becoming "stranded" in the world of the living, with their connection to the other side maintained through an umbilical cord.
BTs can be perceived by individuals suffering from DOOMS, or via a trance connection to bridge babies, which exist between the worlds of the living and dead. The extent to which DOOMS sufferers can interact with BTs varies; Sam is capable of sensing BTs, but cannot see them without the assistance of Lou. Fragile, meanwhile, is capable of seeing BTs without aid. With a DOOMS level of seven at minimum, Higgs is capable of not only seeing BTs, but also controlling them at will.
It is safe to assume that Gazer-types are all the result of human deaths, but very little is explained regarding the nature or origins of the larger BTs. Higgs' Colossus, however, would seem to be an entity that is entirely of his own creation and under his control, suggesting that it is at least possible for BTs to arise from sources other than human death.
A very rare case is a BT baby connected to a living woman. This was the condition of the scientist of Bridges Målingen (AKA Mama) whose baby dies in childbirth while still connected to her by the umbilical chord. The result was that Målingen has to live isolated to avoid any contact with another BTs. However, her BT baby by itself was inoffensive and had some curious behaviors like feeling hungry and sleeping.
An intriguing aspect about this case is that Mama was technically dead; She didn't have pulse and her skin was cold all the time, but was still able to talk, think and interact like any other person. Somehow the BT baby kept her in a limbo between the world of the living and the beach. She can even produce milk even though her baby cannot drink it.
Sadly Mama dies shortly after deciding to cut the link with the baby for various reasons. But curiously her body doesn't necrotize, because the connection with the BT baby filled her cells with Chiralium.
Another curious case is Cliff Unger (AKA Combat Veteran) who having died, returns to life in human form and with improved abilities, as well as being able to control ghostly entities remotely.