Easter', as in the celebration of the death and resurrection of Jesus, was celebrated as Passover in very early Christian times as it had been celebrated for many centuries before. Passover is celebrated in the springtime. As pagan rituals had always occurred during springtime, for the obvious reasons of wishing to celebrate rebirth and fertility, it seems inevitable that these would have collided with the Christian church. Gradually the celebration of Passover and the pagan rituals melded together, and over time turned into what is now celebrated as Easter. The Venerable Bede, in 8th century Britain, refers to the Easter festival and states that the name comes from Oestre.
Rabbits are prodigiously fertile, and unsurprisingly have often been regarded as fertility symbols.
Eggs are another obvious sign of fertility, and brightly-colored eggs have been exchanged as spring gifts in a wide range of pagan cultures, including Celtic Britain. Oestre was supposed to have been approached by a gigantic bird that wanted to be a rabbit, and when she turned him into one, in his gratitude he laid eggs each spring as a ritual offering. Hence the Easter Bunny. It makes perfect sense that animals and plants have become fertility symbols over the centuries. And we still use them.