From Bunni Wiki
Deer are wild creatures that occasionally appear on your island. Their sole purpose is to consume. Their preferred food is fruit fallen from trees, but they will also eat all sorts of flowers. When they are fed, or if there is no food left on the island, they will sleep.
How deer appear
Deer are guaranteed to appear when you start the Destroyer Of Deer quest from Coriander. Other than that, you can obtain deer by shaking fruit trees: deer will just pop out(random, small chance). Furthermore, when your workers destroy fruit trees, there is a chance that a deer will appear. Finally, deer may occasionally appear on your island for no apparent reason.
How to get rid of deer
Deer are generally viewed as unwanted creatures, since they consume valuable resources intended for your workers. There are two ways to quickly dispose of deer. Using a bomb will get rid of all deer and monsters in the explosion's range, but it will also turn every object and creature within its range into bubbles. Bombs can be bought in the Silver Store.
A much more efficient way of disposing deer is by using the special deer bomb. This explosive will also get rid of all deer and monsters in the explosion's range, but it doesn't affect any other objects or creatures. Deer bombs can be obtained from the Bronze Store.
Biddi Attenborough's Guide to Having Fun with Deer
If you wish to keep the deer on your island, it's recommended that you contain them in one isolated area. Create a garden with flowers for deer to eat and make sure that the island has no flowers elsewhere. Your workers have to be fed by shaking fruit trees. Be aware that flowers consumed by deer do not leave bubbles behind, so you will have to keep buying new flowers to keep the deer well-fed. Also make sure that the deer are fed before shaking trees, otherwise they will be attracted to the fallen fruit.
You can take advantage of a glitch in the game to make deer swim out to sea:
- have trees on the southern side of your island
- draw deer to these trees by shaking
- wait till the deer are standing on the seaward side of the tree
- save game
- reload game and deer should now be floating out to sea.