In a campaign I am running there is a monster fighting arena where people can bet on the fights

What are some ideas for monsters that could fight in this arena

Really depends on the setting the arena is in. How powerful are the people that run it? Which creature's could they reasonably source. Is it meant to be monsters vs monsters or do you get gladiators in there etc. If you can answer that then it becomes easier to pick. Here's some general ideas (I did something similar a while back)

Lizardfolk (You could have them as gladiator esque combatants)

Orcs (Similar idea to Lizardfolk)

Dinosaurs of some kind (If they're in your world) they can be fun creatures to fight. Think star wars attack of the clones on Geonosis (Ik they're not dinosaurs but still)

Giant Boars

Polar Bears

Demons (this can be very fun if it fits with the context)

Displacer Beasts (classic)

Owlbears

Elephants

Giant Crocodiles

Vampire Spawn

Cyclops

Giants

Rocs

And if you really wanted to go crazy what about a Narzugon who was tricked into a pact to fight in the arena forever?

I realise I've just listed a bunch of monsters and don't know how helpful that is but hopefully it helps a bit.

Thank you these monsters seem good for arena monsters

Almost anything, though it's presumably entertainment so I'd avoid monsters that rely on stealth or subtle abilities -- watching someone fight an invisible stalker is probably not very exciting.

Lore-wise there might also be other considerations. For example, if they are there against their will, things that can teleport don’t make much sense, because why didn’t they leave? Ditto flying, unless you have some kind of roof over the arena. 
And why haven’t they turned on their captors? Those kinds of questions. So in some ways beast and maybe aberrations or other things with a low int can make sense.

Avoid anything with the Siege Monster trait, unless you plan to have the monsters be subjected to some sort of magic restraint to prevent them from trying to escape.  Also avoid monsters with significant ranged attack or spell options unless you want to explain why they don't massacre the crowds.

Trolls are always a good choice: they're stupid and aggressive so they'll always fight and they're not going to be all that good at trying to escape, and their regeneration means that they can always bounce back from whatever damage they take.  Anything with a "flashy" special ability is sure to be a crowd pleaser.

I always thought the Hydra would be a great arena monster. Lots of flash and spectacle, and you can modify it by starting out with more or less heads/health depending on player level. 

If you could block a guardian nagas speech and block it into an unclimbable area it would make for a good monster due to its innate ability to revive itself and no non spell ranged options that wouldn't be blocked with its mouth

I once ran an arena flight for my players. It was a very old dwarf they met in a tavern the night before. 

They had all kinds of fun that night. Including a fun little bar brawl. They quickly became friends that night. 

My players were surprised to see that same dwarf in the arena. Floating around him was 5 mirrors. You've already guessed what happened. 

The dwarf learned from the players in that tavern brawl. So at the start of the arena battle the mirrors were introduced. Walking out of each mirror was a replica of each player. So they went into battle with themselves. 

I didn't just use their character sheets for my players too battle. I also used their backstory against them. So it was both physical and psychological that they had to overcome. 

Was a very fun battle as for a good portion of the story they were the monsters at that point in time.