I learned snails and slugs can apparently eat rat poison and will get fat on it instead of dying so i would like to present: giant fallout radsnails
Like, brahmin sized. Very slow but relatively unstoppable.
Also, more than two eye stalks.
Some poor settler tries to poison the molerat nest down the hill but instead they attract a giant gooey glow in the dark radsnail that will not leave
The radiation made them much more intelligent than expected, equivalent of the one of a trained dog. Their shells are as resistant as a mirelurk carapace, often littered with vines, mushrooms and vegetation.