Not brand-new, but site search doesn't turn up the exact name (without spaces) so let's do the formalities. Yet another search engine jumps on the Mobile Friendliness bandwagon:
Mozilla/5.0 (iPhone; CPU iPhone OS 8_1 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) Version/8.0 Mobile/12B411 Safari/600.1.4 (compatible; YandexMobileBot/3.0; +http://yandex.com/bots)
Logs suggest they first rolled it out in late December, experimented for a few weeks, and then started using it on a regular basis in early March. Same IPs as ordinary Yandex: primarily 100.43.blahblah with the occasional 141.8.blahblah.
Their own info page, in the section about robots that crawl for purposes other than indexing, says:
YandexMobileBot downloads documents for analysis in order to determine if their page layouts are suitable for mobile devices.
As might be expected, they've picked up a lot of scripts and stylesheets along with pages. Also, inexplicably, at least one .ttf (embedded font).