If animals played April Fool's jokes on each other, it would be by using mimicry and camouflage.
One of the great masters of tricking other animals is the Indonesian mimic octopus:
Camouflage is widespread in the animal kingdom, but here's one I particularly like: the pygmy seahorse.
The tiny creatures (no more than an inch long when fully stretched) were first discovered by scientists examining some coral they had in their laboratory. Imagine their surprise!