Well its been confirmed that the death rider steeds are indeed partly bionic.
The weird hooves are mechanised and the legs have very subtle mechanics if you look closely. I'm really glad that they took this route as its always been a big part of the background.
It also seems according to those on warseer (insert dose of salt as required) that the krieg list has the options of taking HQ on steeds. So it looks like you could have an all krieg death rider force. Much in the Dark Angels deathwing belial example.
A lot of people on warseer are complaining about the price of death riders, Personally i think its quite reasonable. 50 pounds for 5, thats only 10 pounds each. 3 Knarloc riders that are roughly the same size are 40 pounds, same too for the mukaal tallarn monster steed things. For that price (10 pounds) you get one fully detailed resin rider and horse. I mean come on GW sell hideously old metal sculpts for the same price now in blisters. its 9 or 10 pounds now for Ezekiel and Azrael and they are 2nd ed minis for gods sake!
I think FW worlds prices are in some areas maintaining a steady level of reason and at least they don't constantly "adjust" like GW.