The Bullethead (Panopliateuthis velos, “armoured squid dart”) is individually minor but attacks in coordinated swarms, which makes it one of the more punishing encounters in the alien-ruins biomes (it has around 49 spawn locations there).
It strikes tail-first — armoured enough to “punch into titanium” — then clamps onto the player or a Tadpole and drains health until shaken loose. Lore notes it prefers to lodge in already-wounded prey and will wait for a victim to bleed out before feeding.
Counterplay: do not fight a swarm in the open. Use terrain to limit how many can engage at once, keep moving to shake off latched individuals, and retreat to a sealed space if your hull or health is already low — that is exactly when they commit hardest.