difficult to handle or manage; intractable (usually referring to a problem or situation)

thistles, thorns, or thorny undergrowth

To cut one's way through thistles and thorns; to clear away brush and brambles.

echinoderm, the phylum containing sea urchins, sea cucumbers etc

a course of action or life journey beset by many obstacles and hardships

To eat bitter fruit and lie on thorns (idiom); to share the hard life of the common people.

a troublesome affair or a difficult situation to resolve

Tsintaosaurus spinorhinus, a 10-meter-long hadrosaur with a single horn on its duck-billed snout

the taxonomic genus of deciduous shrubs in the family Elaeagnaceae, commonly known as sea-buckthorns