Feral trooper ghouls are former ghoul NCR Troopers that have become feral. Gameplay-wise, they are similar but not identical to normal Feral ghouls, the differences being that they still wear their NCR Trooper armor, which makes them harder to kill, and will sometimes attack with Combat knifes (Other feral ghouls never use weapons). There is also a Glowing one version known as a Glowing trooper ghoul.
They are mainly found around Camp Searchlight, where a freak accident caused all the NCR Troopers there to turn into these monsters, except for Private Edwards, who can be found inside the only accessible house in the camp.
The troopers that attack with Combat Knifes are not actually Feral Ghouls, but Regular Ghouls that are set to be Hostile. As such, their knives and NCR Combat Armour are lootable (unlike trooper ghouls, who's armor is not lootable).