Changing a player to a model?

Well, here are the problems. 1.) Character might not be loaded yet 2.) Character's body parts will fall off. 3.) Character is an Object value.

To wait for the character to load you must do this:

local character = plr.Character
if not character or not character.Parent then
    character = plr.CharacterAdded:wait()
end

To make the model stick you must make joints after everything else:

wait(2)
local plr = game.Players.LocalPlayer
    plr.Character:ClearAllChildren()
    game.Workspace.Chicken:MakeJoints()
    plr.Character = Workspace.Chicken
--Will Break apart

wait(2)
local plr = game.Players.LocalPlayer
    plr.Character:ClearAllChildren()
    plr.Character = Workspace.Chicken
    plr.Character:MakeJoints()
--Will not

Characters can't change value:

game.Players.LocalPlayer.Character = "Hi"
--Will error

The reason you cant move is because you don't have a 1.) Humanoid 2.) Animation Script 3.) Other vital movement stuff. This is all because you cleared the character's children.

If you want, you can use this script:

game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        player.CharacterAppearance = "http://www.roblox.com/Asset/CharacterFetch.ashx?userId=13667373&placeId=0" --This is to look like me! Replace the "13667373" with a players ID.
        player:LoadCharacter()
    end)
end)

Now just make a second roblox account and give it clothes to look like the chicken. Then you can replace "13667373" with the new accounts user ID.

If the chicken is not the same shape as a normal player, you can do this:

game.Players.PlayerAdded:connect(function(player)
    player.CharacterAdded:connect(function(character)
        character.Torso.CFrame = CFrame.new(character.Torso.CFrame) * CFrame.Angles(5,0,0) --Or something
        character:MakeJoints()
    end)
end)

Some of these scripts are just ideas.

Hope it helps!