Are there terrestrial animals with largely elastic skeletal systems?

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I'm not a biologist, but if I understand your question correctly, you are basically looking for land-based invertebrates that also lack a hardened exoskeleton (like insects). This would basically consist of small, soft animals like snails, slugs, leeches, tardigrades, and tons of different types of worms.
The reason that you don't see large examples of this in land-dwelling creatures is that skeletons or exoskeletons become way more necessary without a medium like the water in the ocean to help support a body. The rigid structure provides an attachment point for musculature to create the mechanical levers we use to manipulate our limbs.
- You've understood it right! I wasn't thinking of invertebrates as I was really curious if there might be something strange in-between, but given the other replies here mentioning the same, I think invertebrates may be the closest known of currently.
- Are worms like fish in that they're not really a specific category of animal? More like an umbrella term or a broad classification?
Would you please elaborate on your question? What do you mean by an elastic skeletal system?
Can you give an example, even if fictional?
- Sort of like you see in cartoons, a terrestrial animal that could be "squished" without breaking any bones and goes back to their non-squished state. It's something I think is more observable in aquatic life, like say octopuses that can squeeze themselves into small spaces.
  
  While there may be examples of what you are thinking of, I think the most common solution developed by evolution is invertebrates without an exoskeleton, meaning no skeletal structure at all.

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