It's the same for wild horses and tame horses:
Baby horse=Foal
Teenage Female(Girl) Horse=Filly
Teenage Male(Boy) Horse=Colt
Adult Female(Girl) Horse=Mare
Adult Male(Boy) Horse=Stallion
Adult Male(Boy) Horse that is "fixed"(Gelded)=Gelding
Typically, a horse is either wild or tame, and there is no middle ground. I suppose you could call it semi-feral, semi-wild or semi-tame. If you mean a horse that lives with humans, but has not been gentled yet, you would refer to it as an unbroken horse. If you mean what is a horse that is half mustang or brumby called, then you would call it a mustang cross, or a brumby cross.
a half wild horse
Cowboys called a half-tamed horse a "bronc." This term referred to a horse that was only partially trained or still exhibiting wild behavior. Cowboys would often work with broncs to fully train them for riding.
a "semi-wild" horse.
A male horse is called a stallion, unless it is neutered, in which case it is called a gelding. Obviously, a male wild horse is not neutered, so it is a stallion.
A mythical creature that is half horse and half human is called a centaur. In Greek mythology, centaurs are known for their strength and wild nature, often depicted as living in forests and engaging in battles.
a wild horses can get up to 7 feet tallWell the tallest ever horse recorded was a Shire called Samson. He stood 21.2 and a half hands ( 7 ft 2 inches)
Feral horse, most likely. Feral means half-wild, as an animal that was once domesticated has gone back into its wild state (but is not completely wild like true wild animals are).
A "semi-wild horse" is a Mustang. They are thus called because of their unusual history.
A hippopotomas means 'River Horse'. But is also sometimes called a Water horse
A group of horses that lives together and forages for food together is called a herd. A baby horse of an unspecified gender is a foal. A baby girl horse is a filly A baby boy horse is a colt. A sub-adult horse between the ages (depending on breed) of 1 year and 2 years is called a yearling. A young female horse is still a filly until she is 4 years old (depending on breed). A young boy horse can be is still colt until about the same age (depending on breed) When horses are grown: A grown girl horse is a mare or a dam (if she has a baby) A grown boy horse who can't have babies is a gelding. A grown boy horse who can have babies is a stallion, stud, or sire. Herds are full of foals, fillies, colts, yearlings, and mares/dams. In the wild, there is one stallion/stud/sire per herd. Other grown male horses are not allowed.
Wild horse (Mustang) groups are called bands in the US, or sometimes a herd of mustangs. In Australia they would be called a herd or mob of brumbies and the same would go for New Zealand or kaimanawa wild horses.