A Spike Pig is a hedgehog that has mutated, it has increased in size up to that of a molerat and due to it's size it's eating habits has become far more diverse, they still prefer insects so if you're near one that isn't too hungry it will leave you alone as your flesh doesn't peak it's interest, if you find one that is really hungry on the other hand it will charge towards you.
Spike Pigs when attacking lower their head and charges towards you with it's spikes.
If hit, it takes away ridiculous amounts of health.
It's easy to avoid though (side-stepping the bastard) but if you're caught by surprise then you'll most likely be dead.
After it has charged and knocked you over it will rush over to you and bite you. When bitten by a Spike Pig you catch a disease which lowers certain stats and will need to be cured at a doctor or by medical supplies.
But the Spike Pig is generally friendly and not much of a concern, it's spikes on the other hand protects their flesh so if you decide to sneak attack critical one for easy exp then think again cause it will be in full rage if you fail to kill it.

The Dusk Creeper is a wolverine.
Due to the radiation they lost all their furr and most of their fat and muscles on their bodies.
They are about the size of a pet cat.
They're skinny, fast, hyperactive, has razor sharp claws and they attack in groups.
These things are horrible to be faced with, they don't have a lot of health so explosives can take care of them real good, but aiming at them with a gun or energy weapon?
You'll miss them, a lot.
Due to their speed and razor sharp claws they are fierce enemies which can quickly take down just about anything they face.
To put some perspective on it, a group of 10 Dusk Creepers can easily take down 2 deathclaws.
"But they are overpowered!" Nope, they're not, as I said, just use explosives or maybe the flamer on them and you'll easily kill them.

The Tyrantula, despite its name, is not actually related at all to the rather infamous spider (other than in possessing some spider DNA). Instead, this eight-legged monstrosity is the product of pre-war biological engineering performed on a small population of fiddler crabs. Spliced with DNA samples taken from ants, barnacles, and various venomous spiders and scorpions indigenous to the West Coast, the resultant abomination is an eight-foot armored shell with two massive claws (one often larger in size than the whole of the body) and between three and four venomous tails. Although they may look menacing, the reality is much worse. Tyrantulas are aggressive, intrusive, and never come alone. Hive creatures, Tyrantula 'footsoldiers' often travel vast distances of land in search of new sources of food and water. They do not travel alone, preferring instead to scout in groups of four to six individuals. Much larger, more terrifying specimens guard the Queen... where she resides, deep beneath the surface in a network of labyrinthine tunnels.

The first records of PL-905x growing in the wild occurred in 2201, when Enclave Scientists discovered a large population of the entity thriving in the ruins surrounding the Environmental Protection Agency. At first, it seemed to be dormant... and the scientists were confident that they could safely study it. However, it became active the moment a culture was taken for anolysis. Almost instantly, a hive of 'infected' ants began launching wave after wave of defenders at the researchers, who were forced to retreat or be ripped to shreds by the creatures seemingly under the sway of the Central Stalk. Although they never did reach the Central Stalk itself, they reported that it would be hard for one to miss... as the fungal towers can stand upwards of twenty to thirty feet in height.