I’m hoping to get my halfling player on one of these soon!

I was given one by another player and purely for aesthetics we're saying it's an Alaskan Malamute. I'm in love. 

Can i as a gloom stalker purchase one of these for a guard dog or perhaps even as an animal companion? 

Dont ask us. Ask your dm. What I personally did though was, since im a ranger, I bought a mastiff as a mount at level 1 then made it my companion at level 3

I have a Gnome Paladin with Find Steed. She summons a mastiff using the spell for her mount.

Is there a way To make character sheets for Someones  That would be incredibly 

Can Mastiffs also be companions and not just mounts?

Are there other creatures that can be used as mounts for halflings and other small humanoids?

I mean, you could use a pony (the other medium option summoned by Find Steed), and basically any medium-sized quadruped is theoretically rideable. For example, a small-size Beastmaster ranger riding a pterodactyl or panther, or a goblin astride a trained wolf. Outside of class/spell-granted creatures, it's basically a question of what animals the DM will let you buy and/or train.

My Kenku Ckeric has a mastiff to protect her from all the cats out there-because they're TERRIFYING!

I'd think so. You dont ride your horse all the time, or at least I didn't. And you'd develop more of a kinship with your dog, so that seems reasonable to me.

I'm about to be in a campaign where I am playing an otter that will be riding a female mastiff named lily!

me too

is there any circumstance that a player or the dm can somehow find a way to transform the mastiff into a shadow mastiff?

I wonder. Can a changeling who changed form into a halfling, gnome or dwarf ride them? For I read that the stats or racial advantages do not change even as a changeling change race. Does that apply to weight as well?

Read in a DND 5E wiki that you can sell a shadow mastiff whelp for 200-500 gold(So if the DM allows it, it could be allowed for one to buy one). So I guess it should be possible to get one. But not turning an existing one into one, sound rather hard. They are suppose to have been created from a sort of demonic possession if I remember correctly, and it would be hard to control the mastiff if it got possessed(I am guessing it would be like a human IRL trying to control a dog with rabies).

Starting over with a natural born whelp of one and train it should be easier, and probably work with the "Animal Handling" skill.....I think?

Why is it not Lawful good? I feel betrayed. Co-president Riegel would not approve.

+1 to all Good Boi rolls.

what is the animal handling roll needed for taming?

Dumb question...as a DM, if my players adopt this doggo they come across (and I hope they will lol), can a companion level up? Or does it stay as is?