They kind of have had bites taken out of them.
These are a result of oxidative damage to the hemoglobin .. drugs/toxins/and some foods in pets can cause these.
The hemoglobin is damaged in the red blood cell from the oxidative damage, and the spleen tries to remove them from circulation. But sometimes, the cell is not completely removed yet, and you see these cells with portions of hemoglobin "heinz body" or the bite cells left behind.
Here's the more complicated explaination off wikipedia:
Heinz bodies are formed by damage to the hemoglobin component molecules, usually through oxidant damage, or from an inherited mutation (i.e. change of an internal amino acid residue). As a result, an electron from the hemoglobin is transferred to an oxygen molecule, which creates a reactive oxygen species (ROS) that can cause severe cell damage leading to premature cell lysis. Damaged cells are cleared by macrophages in the spleen, where the precipitate and damaged membrane are removed, leading to characteristic "bite cells". The denaturing process is irreversible and the continual elimination of damaged cells leads to Heinz body anemia.
There are several pathways leading to the hemoglobin damage.
In this case : it was caused by the onions. They do that for some reason. And, more likely in cats than dogs, but happens in both.