Does AEM 6.1 provide an out of the box captcha component for adaptive forms?
I couldn't easily find anyone (even under /libs/fd/af/components), but this message seems to indicate that there is one...
There is no OOTB captcha component available in AEM Forms as of today. There is a captcha component available OOTB as part of AEM Foundation component as seen here  in CRX/DE.
You can anytime create a custom component in AEM forms for your usecase.