There is no”correct” adjective.

Animosité is not a nominalisation of some hypothetical adjective, but a direct borrowing from Latin animositas. The same root eventually gave animal and animus, but despite being etymologically connected, these words are semantically unrelated to animosité.

I totally agree with @Circeus, regarding the fact that there is not any adjective etymologically corresponding with animosité.
And this precisely answered the OP since it stated "which use the same stem".

But it’s always possible (even if also always debatable 🙂 to propose an adjective closer corresponding from the viewpoint of the significance.

Then fielleux is not appropriate : it partly brings the sense of hypocrisy, while animosité always assumes a frank behaviour.
This is clearly attested by all the definitions found in the TLFI:

Manifestation d’un sentiment vif et parfois durable de malveillance systématique

Emportement violent dans une discussion, une polémique, un affrontement

Even when used figuratively:

Grande vivacité

So these adjectives might be better candidates (though they don’t fully yield the combination of nuances of animosité, and should be chosen depending on the context): vindicatif, malveillant, emporté, haineux, agressif, hostile…