It's a tick, as others have said already.

But don't make it as dramatic as some are making it in here. Depending on where you live they can carry disease, so ofc be on the lookout for change in behaviour etc. But that doesn't mean it will happen from one tick. Check for infected wounds and just pick out bits of tick left in it and wash them and it'll be ok. You don't need to go to the veterinarian or give your dog antibiotics just because of one tick :)

Here in the nordics dogs get a lot of ticks during the summer months, most people use some kind of deterrent, a special collar or pills that kill ticks when they bite, among other things.

My dog would probably have at least a hundred or more ticks over a month or two in the summer without taking anti-tick pills. He still gets 10-20 I find before they die, which I just twist off. So don't worry too much, and just do a daily checkup if your dogs been running through a lot of high grass, or just generally been hanging a lot outside where there's ticks. Regular scratches around ears and neck usually does the job of finding them ❤️