What I expect to happen: passive mobs, like chickens or horses, will avoid going on cacti.

What actually happens: Chickens and horses, even on lead, can keep going on a cactus and die after several tries.

How to reproduce: just observe animals on deserts. It is a good idea to spawn them from spawn eggs (or submit them with a code).

Note: when hit with anything (like cactus thorns), the animal should flee. Their intelligence in the game is so low that they are prone to suicide. Let creepers die on cacti but why should horses?