These are just rats plucked straight from a Chicago alleyway. 

Would a Goblin be able to ride a giant rat or is that too much weight

The rodents of unusual size? I don't believe they exist.

In The Icewind Dale module they describe them as hafling sized

can you eat these

To make these critters a little less redundant without changing their stats nor abilities, call them giant naked mole rats.

Rodents of unusual size.

 Im gonna make it where the giant rat is large enough for a goblin to ride it because I think my party would like that

No, creatures can only ride other creatures that are one size or larger. Hence why a horse is Large. Gnomes and halflings could ride a Medium-sized creature like a pony, but a giant rat is as big as they are.

hello to all other dms putting these against your first level players in a tavern

Mastiffs should have higher HP. Once, my party was fighting a three goblins and the leader's mastiff. One of the players used Burning Hands, and intentionally angled it to hit the mastiff. The mastiff was the only one to fail the save. He dealt 10 fire damage, killing the dog.

Quote from lilmanjosh >>

Mastiffs should have higher HP. Once, my party was fighting a three goblins and the leader's mastiff. One of the players used Burning Hands, and intentionally angled it to hit the mastiff. The mastiff was the only one to fail the save. He dealt 10 fire damage, killing the dog.

Fair enough, they mostly have low hp to help classify them as weaker than wolves while keeping wolves as a low level threat. I generally think of animals' low hp as meaning they lack the skill to protect themselves from attacks from actual weapons, but there's nothing to stop us from increasing the health or strength of anything we want to make tougher. Against tough enough characters its fine to give things like giants, ogres, bears and stuff more strength or health to make it clear they're stronger than a human can be. 

No, only Tiny creatures can ride a Small creature.

Quote from EkaterinaQuarantine >>

I adopted one in my recent campaign. His name is Renald :3

 Same! I named mine leo and Lata

Probs a new question

How do I make this a pet or give custom stats

Got approved to have a pet 10'0 rat in the next campaign

It's going to be fun

Wait until Bruno finds a few of these...

Giant i'd reckon

bro i just realised theses things are bigger than halflings

my companion is a large cloud cloud rat. He is well trained but aggressive towards other rats