Maybe for an UU.
It is a catapult with a very weaker attack, compared to normal onagres. But, the projectile should have an additional effect. Here my suggestions:
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It could damage the garrisoned units inside Rams, towers, ships and buildings. A normal onagres would destroy a town center in a much faster time. But, a corpse catapult would harm the villagers inside the town center with every shot, killing them after a number of shots. That forces your rival to evacuate them before the TC destruction.
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It could create an area of effect with damage on time. But this could be complicated, and it would change a lot of the gameplay.
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It could poison the units attacked, with a small damage on time for the next minute. The units could garrison to try to survive. But normal onagres would just kill the units, which is better.
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It just has a bonus against Calvary and ships. But the normal onagres are that strong that it don’t need any bonus against those units.
I consider the first option the one that gives the unit an original niche, that can be useful in raids.
The other options would be cool if the corpse catapult had a cheap prize, not like the onagre. It could even be considered a trash siege.
There were some civs that used biological warfare in this sense. Turks and tatar are good examples.