The Unicorn is completely immune to the Poison status, but is still vulnerable to the Poison element damage (such as dealt by Poison Strike, Strangulate, or some creature's melee, such as Putrifier). The Unicorn can also attempt to cure you if you're poisoned and at low health.
The Najasaurus is COMPLETELY immune to Poison. As in not only is it immune to the Poison status, it also has 100% Poison Resist. The Najasaurus is also completely overstatted for it's slots. A freshly tamed, 2 Slot Najasaurus can have 750+ Health, up to 300 DEX (anything above 190 is a waste) and up to 171.0 Resist after tame. They come with the Poison ability, and that's it, so they're pretty customizable. I have 2 Najasaurus. One with FWW+Poison Breath, that i use for AoEing down spawn at Champs, and one with AI+Poison Breath, which i mostly use for tanking Putrifier when gathering Paroxy keys. Between Consume Damage and 100% Poison Resist, the Putrifier can't even scratch my Najasaurus.