When dealing with animal or very low intelligence creatures I have a simple rule - aggressive herbivores attack the largest target, predators attack the smallest.
If the wyvern is going to swoop down for a game of grab 'n' drop then Mr. Halfling best beware.
Bob the Barbarian gets to find out what a real bull rush feels like.
And they go after their chosen target until the target either runs away, does enough damage to be obviously not worth while (more for predators than herbivores - a bull will keep at you, even if wounded), or goes down.
Aggressive herbivores are bad news - Cape Bison and hippos are the most dangerous creatures in Africa, other than humans....
The Auld Grump