Check the event message first
“Thorn monster” does not identify one precise enemy well enough to choose a safe counter. Start with the message that appeared when the threat spawned and the name shown by the board or collection.
A plant is one possibility
The Carnivorous Plant spawn rhyme begins Hidden in leaves, it lays in slumber. Match that message to what appeared rather than treating a spiky silhouette alone as proof.
A plant-like creature, spikes in a trap room and a hazard attached to a door are not interchangeable situations. A response for one can fail against another.
Before using a scarce die
Keep space, watch what triggers the threat and compare it with the event you just rolled. Do not assume fire is a guaranteed weakness purely because the creature resembles a plant.
If you are identifying a different enemy, the Sandworm, Mummy and Snowman pages describe their own warning signs. Do not apply those instructions to an unidentified creature.