NormalUndeadUndeadMediumLv 42

Pirate Skeleton

≈500per thousand kills

fresh monster

≈3.6per thousand kills

near level 10

Where to find Pirate Skeleton

If you have never farmed this monsterIt is at its peak: ≈500 research per thousand kills, against ≈3.6 once it nears level 10.
If it is the only bottleneck of your lineEvery kill is worth 7 to 60 times an ordinary one — it is unlocking the bonus for all five.
If it is ahead of the other fourIt is the worst target on the line, and it loses on both axes: it unlocks no bonus, and its individual research per kill has already fallen with the level.
If the line is incompleteIt pays no bonus at all — the kill is worth only the individual research.