JKR's own drawing of the map of Hogwarts indicated that the forest is bounded on at least two sides by the Hogwarts wall, a road and on one side by a train track
We also learn that the forest "extends" in a broadly North-easterly direction.
The Forest is described by JKR (in another sketch map) as "Massive" and stretching out of sight.. You can also see ...
At the end of Deathly Hallows Hagrid explains how he got back in to the school grounds:
Smashed our way through the boundary by the forest, Grawpy was carryin' us, Fang an' me.
I take this to mean that he did not simply come in through the forest with no physical obstacle. It would either mean that there is a boundary (wall) preventing one from entering ...
"in 1750 Clause 73 was inserted to the International Statute of Wizarding Secrecy, to which wizard ministries worldwide conform today: Each wizarding governing body will be responsible for the concealment, care and control of all magical beasts , beings and spirits dwelling within its territory's borders..."
(From Fantastic Beasts and Where ...
Flying creatures can leave.
We have at least two examples of winged animals leaving the Hogwarts forest: Buckbeak and the Thestrals.
Buckbeak flies away with Sirius:
He squeezed Buckbeak's sides with his heels. Harry and Hermione jumped back as the enormous wings rose once more... The hippogriff took off into the air... He and his rider became smaller ...
The creatures that weren't clearly there because of Hagrid weren't particularly dangerous.
Centaurs and unicorns are two of the creatures confirmed to be in the Forbidden Forest that might not have been because of Hagrid. Both of them are rated XXXX by the Ministry of Magic, but because they should be treated with respect, not because they're dangerous.
There are a few times in which the students enter deep into the forest, namely the Spider scene in book 2, the Unicorn scene in book 1, Gramp in book 5, and the death scene in book 7. I get the impression each time that they are walking for at least a half hour, and that the final two locations are different.
Also, we know that there are many creatures ...
No in-universe answer I could find so far, but out of Universe, there are 2 possible locations that are associated with Forbidden Forest:
530 acre Black Park
In the movies: In Sorcerer’s Stone, Black Park is the backdrop to Hagrid’s cabin (Forbidden Forest). In the Goblet of Fire it is the setting for the night dragon scene in the woods when Harry first ...
I would like to think that the creatures in the forbidden forest are not confined in Hogwart's as Buckbeak/Witherwings was able to fly out. This means that there are no magical boundaries that prevent the animals to come out.
In my own opinion, the animals stay in the forest as it is safe and a perfect place for living as a creature
I think the premise of the question is not necessarily correct. There are not actually that many magical creatures that live in the forest (at least that we see throughout the books).
The following magical creatures are seen in the forest:
According to Fantastic ...