One example would be a worm, there are less fossil records, if any at all, of worms due to their completely soft-bodied form. They have no bones that could be preserved through time, and when decay happens, they mix with the soil and completely decay,
Another example would be Jellyfish, like the worm, this is a completely soft-bodied creature, and dead jellyfishes also decays unto the sea directly living no fossil behind. Fossils as one should remember are from the bones or shells that have been petrified through time, and has not decayed completely.