I am trying to come up with a creature that is worshiped by a cult for my character, and because I am not particularly creative, I decided to rip off Team Magma from third generation pokemon.

For those unfamiliar, the cult worships the legendary pokemon Groudon, a giant lizard/dinosaur like creature with abilities centered around fire and lava.

Is there a creature that already exists in 5e that can serve as an equivalent, or do I need to come up with one myself?

Ideally the creature would be able to create land, which would be a motivating factor for worship by the cultists.

Strength would ideally be something strong enough that it would be unlikely to die in combat to PCs or NPCs, however it does not have to be some world ending force of nature that could single-handedly destroy the realm if it desired.

An Ancient Red Dragon.

You won't find a much more archetypal "giant lizard/dinosaur like creature with abilities centered around fire and lava" than a red dragon.

As a lair action, an ancient red dragon can cause a magma eruption:

Magma erupts from a point on the ground the dragon can see within 120 feet of it, creating a 20-foot-high, 5-foot-radius geyser. Each creature in the geyser’s area must make a DC 15 Dexterity saving throw, taking 21 (6d6) fire damage on a failed save, or half as much damage on a successful one.

When that magma cools, new land would be created. Through repeated use in the same or close by areas, it could add up to a significant landmass.

In addition,

Small earthquakes are common within 6 miles of the dragon’s lair

These earthquakes could also cause new landmasses to appear out of the ocean.

So you've definitely got the fire and magma theme going, and an (admittedly somewhat contrived) ability to create land. A red dragon technically breathes fire, but it's not a huge leap for the particular red dragon your cult worships to be breathing magma instead. In terms of the game's mechanics, there would be no real difference.

And of course, an ancient red dragon is unlikely to be defeated by any but the most powerful PCs or NPCs.

Salamander

This may be what you're looking for. While it doesn't breathe lava, it does have a heated body that deals fire damage if something touches it that could be fluffed to be lava coming out.

Your goals:

1. Lizard form - Check!


2. Lava relationship - Check!


3. Breathe Lava - No check. But the heated body may be a good substitute


4. Creating land - No literal check, but cultists may not understand what the Salamander is actually doing. If they know that it's moving around in magma, and as the magma hardens it turns to land, they may think there's a connection there.

Magma Mephit

Whilst this isn't quite right, since it's made of magma rather than being a lizard creature, it does give you a starting point to potentially reflavour a creature to fit your own needs.

For example, you could give the Death Burst and Fire Breath traits (scale the damage depending on the CR of the creature you want to add these to) to another creature, such as a Salamander (as per NautArch's answer) or maybe a Giant Lizard or even a T. Rex (if you wanted to keep to a "lizard" aesthetic, although that would need to be tweaked further, such as changing it's type to elemental and giving it immunity to fire, maybe borrowing the Heated Body trait from the Salamander too).

If ever you need a creature which doesn't exist in D&D, but there are similar things that do exist, you can always tweak them or combine them to suit your needs without having to actually come up with a whole new creature yourself.