get rathian plates from the tail of G-rank rathian (any colour but the greens tail I think has the highest %)
can get plates from any normal rank rathian when you, break head, carve tail, capture or drop (all have lower chance than the g-rank tail)
Also G-rank rathian replaces the plate with ruby for some of the rewards
I.e. If you break the head of a G-rank Rathian you have a chance of getting a ruby but a normal rank one will give you a chance of a plate