How does the magnetic field affect creatures on a planet?

If that earth like planet has much stronger magnetic field than ours, then they'd have very different life system then ours. That planet life would have a very small chance of making it to the dryland . If some lives makes it to the dryland it would most likely be a shell based or cuticle based life form.

The climate would also change , there would be more charge present in planets atmosphere, so lightning storm would occur more frequently .

Many mutations that result in evolutionary changes are the result of charged particles and other radiation that make it through earth's magnetic field, so a planet with a stronger field would have a slower rate of evolution. The creatures that did evolve would likely be more sensitive to radiation.

The magnetic fields have another effect you did not mention. All life senses this to a small or great extent and it has strong effects upon migration and breeding, and navigation. If the magnetic field were markedly changed, animals may not be able to navigate on land, and most sea life would be even more strongly affected.

If they cannot find one another or their spawning areas, species could well die out.

Dinosaurs are nonetheless approximately right this moment, they're purely talked approximately as birds now. The consensus is that climate substitute (subsequently international cooling) related to the time of the result journey that brought about the iridium layer alongside the KT boundary furnish a variety rigidity that favoured the extra avian dinosaurs.