I don't think we need an achievement for every mount we learn
I don't see how the sky golem is special. It is one of the easiest and cheapest mounts to farm.
The bike + mammoth were the major gold sinks of WotLK that's why they had an achievement.
The sandstone drake was the major gold sink of cata (and a crappy one honestly).
The yak, the two rockets and the JC tigers were the major gold sinks of MoP.
If any mount deserves an achievement it's those rockets or the onyx tiger.