The Collaborative International Dictionary
Horntail \Horn"tail`\, n. (Zo["o]l.) Any one of family ( Urocerid[ae]) of large hymenopterous insects, allied to the sawflies. The larv[ae] bore in the wood of trees. So called from the long, stout ovipositors of the females.
Wiktionary
n. The wood wasp.
Wikipedia
Horntail or wood wasp is the common name for any of the 150 non-social species of the family Siricidae, of the order Hymenoptera, a type of xylophagous sawfly. This family was, until recently, believed to be the sole living representative of the superfamily Siricoidea, a group well represented in Paleogene and Mesozoic times, but the family Anaxyelidae has recently been linked to this group. The last tergite of the abdomen has a strong, projecting spike, thus giving the group its common name (the ovipositor is typically longer and also projects posteriorly, but it is not the source of the name). A typical adult horntail is brown, blue, or black with yellow parts, and may often reach up to 4 cm long. The pigeon horntail (Tremex columba) can grow up to 5 cm long (not counting the ovipositor), among the longest of all Hymenoptera.
Female horntails lay their eggs in trees. The larvae bore into the wood and live in the tree for up to two years, possibly more. They typically migrate to just under the bark before pupation.
The spiral groove on the ovipositor is visible on the photograph but not easily to the naked eye.
Usage examples of "horntail".
Knowing what was left, Harry put his hand into the silk bag and pulled out the Hungarian Horntail, and the number four.
And there was the Horntail, at the other end of the enclosure, crouched low over her clutch of eggs, her wings half-furled, her evil, yellow eyes upon him, a monstrous, scaly, black lizard, thrashing her spiked tail, heaving yard-long gouge marks in the hard ground.
Quidditch match, and that Horntail was just another ugly opposing team.
Now that the Horntail had been taken away, Harry could see where the five judges were sitting - right at the other end, in raised seats draped in gold.
Harry set his tiny model of the Hungarian Horntail on the table next to his bed, where it yawned, curled up, and closed its eyes.
He was very grateful that Swedish Shortsnouts did not have the kind of tails Hungarian Horntails had.