A deer's biography can be pretty much anything you want it to be. Just create a bio page and describe what your deer is like. There are some pre-made HTML/CSS templates if you'd like to make it look fancy, too. Look around at the other biographies out there and see what you like. As long as you don't copy things, it's okay to find inspiration in other places.
This is the biography of my very first deer, Rire. I also have Rohit, Laurel, and Sedirea.

Herds and territory are things that have been discussed and turned down many times over the 10 years the game has existed. The creators' original intent was to bring people together, not to divide them, so while it's fun to play that way every now and then, it never has and probably never will be an official part of the game.

Deer do migrate in the real world! There are some fascinating documentaries on the long journeys they make, and all the hardships deer have to face - particularly as their migration ranges are being altered more and more by human activity. They have to cross highways and scale fences in addition to the great distances and mountain ranges, etc, that they have always had to conquer.

I used to read the Warriors series, but it's been a long time. I stopped partway through the Power of Three arc, if I recall correctly.

^ A migration would be fun in roleplay, but kinda hard to pull off in the game. If enough planning is put into it I don't see why not.

A lot of your ideas kinda sound like some new features they're putting in Wolfquest, which is getting a big update like TEF! I played it before and it was fun. You can still roleplay and make your own characters, but they're wolves instead of deer. They're trying to make the game as real as possible too! Here's their youtube channel. It costs $10 to play the old version, so you're probably gonna have to ask an adult to buy it for you, but I really recommend it.

You can look at other player's bios (make sure to check their disclaimers!) to get an idea on how to make your own character, but don't just copy the character and call it your own. I use this big developer a lot and it helps me make my own characters when I don't really know where to start.

e: Also, I think I read Warrior cats up until The New Prophecy, which was ... ages ago but I still remember how the world works.

ive seen the movie of the guardians of gahool but I never read the book before( and I just finished midnight and moonrise in the new prophecy series if you wanted to ask me what book I was on but idk what the third book is called?) also ive herd about the wolfquest game but I am not allowed to play games that you have to buy to play like steam games like towers unite or rocket league but ive seen videos of people playing steam games. like markiplier jacksepticeye and cryaotic and yes I watch them sometimes( I also watch m k ice and fire he does no commentary gameplays so check him out too) also for the remake they should add hoof prints so we can see whos who( if its a adult deer the hoove marks are big, if its a fawn there tiny or small.) and for the wolfquest thing that costs $10 you were saying that's for the new one the old one is free to play( you know where you find a mate but to play multiplayer you have to wait a week or something like that to play in multiplayer if you sign up for it that is.)