Those are Imperial Knights by the way, not Titans.
Somewhat smaller than the Warhound, the scout and smallest Titan, they are single pilot mechs with nobles (as in chivalric households) as the pilots, they are fielded by the noble Houses as allies to the IG/Marines/Titan Legios or as Houses within the Mechanicus as well. There are also Freeblades which operate independently. If theres not a thread on the iPad Freeblade then go check it out, its a f2p on rails shooter. The entire Knight franchise turned up a few years ago, and popped into the fluff as if they had always existed.
The Eldar mech was a Wraithknight, which is a large walker mech piloted by twins, although one has to be dead (dead Eldar souls end up in spirit stones and can pilot mechs and things) and is a bigger version of the Eldar Wraithlord dreadnought from previous games.