Toptal malicious cookie on the WordPress page?

I have a really connected problem. So – I have a WP template written by myself; I do not use any add-ons (except ACF for coding), but I get a cookie called "" if I visit my own site. I can erase the cache and everything. Visit it again, it appears. I visited all my template files, to be sure, it's not from there. This problem really drives me crazy. It does not happen with other sites, what could it be?