Child and toddler helmets have appeared with rubber animal ears and noses mounted on them. Some go well beyond that, with high ridges, horns and other projections. The decorations are cute, but they must detach readily to avoid being a hazard in a fall.
The CPSC child bicycle helmet standard outlaws projections of more than 1/4 inch (7mm) that do not collapse or break away readily when tested. I remember that once, we take a sample to test but it did not passed because of projections. CPSC wrote that into their standard because lab studies have shown that projections can cause a helmet to snag in a fall and increase the strain on the rider's neck and to the rider's brain. In other words, when a rider falls, it is necessary for him or her to slide on the road, if it is stopped promptly, the neck will face an intense rotation force and thus cause tremendous damage to the rider. The animal decorations usually collapse partly, but the remaining lump for many is higher than 7mm. Though most of the animal projections are soft rubber, but some have a harder core, and many do not readily detach. Therefore, it will prevent your head from sliding and bring hazards.
Disclaimer: nobody has gathered statistics on whether injury rates are higher in animal helmets. Our comments above are based on science and the knowledge that sliding on impact lessens injurious forces to the head and neck. A helmet with animals on the exterior is always better than no helmet at all.