Magma golem

It would be made the same as iron golem, but insted of iron you would be using magma blocks and insted of pumkin on top, you would be using shroom light.

It wouldnt look like retextured iron golem, it would have its own model. 

It could swim in lava without taking any damage, but when in water, he would take damege.

After killing, it could drop magma block or some nether plants. It could potenitally drop new items/blocks.

It would attack piglins and maybe other creatures in nether.

In other dimension then nether, it wouldnt be as strong as in nether, but you could bring him to overworld and end.

It could potentially wait near the portal when you leave nether, or wait in ur nether base i you right click him. Maybe it would sit in place, just like dogs do.