A baton (as it's used in the bushcraft world) is very useful for dividing up frozen things before defrosting them. You use it in combination with a meat cleaver or your cheapest chef's knife.
Honestly, a random log or 2x4 works great, especially because the baton is likely to be slightly damaged over time and these materials are cheap and easy to replace. But in some kitchens, you might want to not seem like a barbarian, so something more polished can be good.
Requirements:
Non-metal — If you can avoid hitting metal-on-metal, it's usually best. This way the metal surface of the knife can't get damaged. Also, some people don't like the metal-on-metal noise.
Heavy enough — You're cutting through hard stuff, ranging from ice at the easiest to frozen leather and bone at the most difficult. A light-weight mallet isn't going to cut it.
The striking face should not become too damaged over time.
Options:
Rawhide hammer — These are junk, the normal ones are way too light.
All-wood mallet / joiner's mallet — These work, but they take up a lot of space and don't exactly fit in the spatula drawer, and they're not as heavy as you need sometimes.
Nylon carving mallet — This is my preference. The nylon feels like it might get dinged up, but it won't split for a long time since the "striking face" is very wide.
They tend to only be available in the 10 oz / 300 g size range, but if that works for you, then great.
Split-head mallet — If money were no object, this would probably be my choice, since it's heavy and the striking face is easily replaced. Unfortunately they cost $70+, by far the most expensive option.
Brass hammer — This shouldn't dent a knife's metal surface (the Mohs hardness of steel is 4.0-4.5, while brass is 3.0), but it makes a distinctive noise that some people don't like.
Lead hammer — It definitely won't damage a steel knife (lead has a Mohs hardness of 1.5), and the sound tends to be far more "dampened" compared to brass hammers. However, there are real concerns about its toxicity.
Safety issues with lead — Handling solid lead isn't as unsafe as people think, and the real danger is if it gets turned into lead dust and is inhaled or ingested. If the lead stays as one solid piece, there is zero danger, since skin absorption isn't a concern. Most likely the biggest hazard here is that dust particles will come off the hammer and get embedded in food — which is a very legitimate concern.