I don't even know how you'd do up a feystag in 4e, since the way magic item powers work have changed so drastically. That one requires some serious pondering...
I love the gambado, but feared it might be so weird that I was the only one. Glad to see that is not true!
I'll do up some giant dragonflies at some point, but I was thinking that I'd go a little higher level and make them paragon-level flying mounts.

As to the spike skeleton, I'll have to look it up- I don't recall anything specifically about it.