Are there other Deathworlders aside from humans?

Oh most definitely. Earth itself rates a 4.5 on the Deathworld scale, and most of that is because of the weird, nasty, and unusually venomous stuff that lives in Australia.

There are level 5 Deathworlders, and humans are about the only ones who can deal with them… one way or another.

And there’s all kinds of Deathworlders from 0.5 to 5 and How The Flakk Did They Evolve?

Interestingly, low-grav species like the Huf’nuf’ruf and the Jelly Dancers count as Level 5 Deathworlders because space is very hostile towards life.