Don't assume just because they are big, they will be slow. They are insanely fast on land and in sea. Their top speed, reactions, attack speed and turning radius are sharper than their teeth and claws. Just because you are on your prized Raptor, sniping from a distance, does not mean it won't reach you faster than you can turn and start to run away.
If you are having a hard time finding spino's in their respective spawn zones (especially Ragnarok), kill all the fish in that area. They share the same spawn capacity and can be out spawned by coles! ( up vote so Rag players know!!!)