I just discovered one on Who Is Online?, but it had no IP address. I've heard of Google bots, and just read their are others.

My Twitter is activated although I don't have an account associated with my forum; haven't activated other social medias yet. Topic'it is activated.

Are there any bad bots that I should be concerned about?

Thanks.

No, there is nothing bad about them. They are simply bots used to index your forum on their search engines.

I forgot to ask before marking this solved - how often do bots visit forums, and do they increase the chance of others finding it randomly when googling for, say, dog forums, or car forums?

If you share content on Facebook, Google+, Twitter, Pinterest, etc., it usually causes search engine bots to index your forum.  So this is one way to increase your visibility on search engines.

They don't have a time of day or a limit to how often they come on to you site they just go by Websites demand
Trust me it's all good and safe so you have nothing to worry about.