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Umbonia crassicornis
Umbonia crassicornis is a common and widespread member of the family Membracidae, and one of numerous species colloquially referred to as thorn bugs. The body length of the adult is approximately 10 millimetres (0.39 in). This is a variable species as to size, color and structure, particularly the pronotal horn of males (which is more angled posteriorly than the females' and often somewhat expanded apically). This tall, essentially perpendicular thorn-like pronotum discourages birds and other predators from eating it, if only by mistakenly confusing it with a thorn.
Thanks! They were in the middle of a forest, and at first we thought they were actually thorns!