Horntail's Wyvern summoning animation?

Can't get any clearer than that :o

This. Just look for that every time you run. As a LOLFP who always cleared the spawn, my best recommendation is going to stand directly under mid-head the second you see that insignia. This allows you to take Wyvern touch damage, and thus gives you crucial invincibility frames when dispelled (if Dark Wyverns spawn) to pick them off.