4-LOM: Looking like an insect-headed protocol droid makes sense when you learn that 4-LOM was in fact once a protocol droid, clearly patterned on an apparently angry looking humanoid insect species in the same manner that C-3PO is meant to represent the humans he interacts with. 4-LOM's programming became corrupted leading him on a path to becoming a ruthless bounty hunter.
Ignore the expanded universe material that suggests that he reformed and joined the rebel alliance for a time, and cherish him as a droid who looks like he means business. As a bounty hunter, his name is supposed to stand for "For Love of Money."
ZUCKUSS: This short, insect-like bounty hunter is a Gand, inhabitants of a gaseous planet of the same name. Offworld, Zuckuss must wear a breathing mask in order to protect him from oxygen. Beneath his cloak he wears battle armor and a computer and sensor equipment.
This bounty hunter who works using intuition to hunt his prey was partnered with Bossk for a time before partnering with 4-LOM.
These two bounty hunters, while of very different physical types, had very similar heads. As you can see by comparing the Zuckuss head mask below, with the image of 4-LOM further above, except for the eyes, their masks like like they were cast from the same mold. Zuckuss has tubes plugged into the two metal holes on the mask's chin, differentiating them a bit more.
This similarity of appearance is what probably led to a further mix-up, leading to more confusion between the identities of the two bounty hunters. For anyone who was a kid when The Empire Strikes Back was initially released you're likely to think of the droid as Zuckuss and the squat bug as 4-LOM. This is because when Kenner released these two characters as action figures, they reversed the names. Following the pattern set by other droids such as R2-D2 and IG-88, when Hasbro released new versions of these characters in the modern line of Star Wars action figures, their names were corrected.
It was also likely this mix-up which led to the two characters being paired up in the expanded universe material.