It seems community feedback was loud and got heard, and will be incorporated swiftly! After seeing the 500 mounts collected achievement reward, the Carrier Yeti, the community immediately commented on how underwhelming it is for such a hard achievement, and we now have confirmation that it will indeed be replaced! 

The old yeti, to be replaced.

Yeti (Source)

Yep, we agree. The yeti will be changed in a future alpha build, though not necessarily the next one. Feedback like this is always useful, and please take this as a reminder that things you see in alpha builds are subject to change.