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No, you can't transfer merits to animals. You can only do that with a class of Petas(hungry ghosts) called the 'Paradattupa-jivi'. Animals at best might hear some Dhamma, memorise it and as a result be born in a better place next life. But that is extremely rare. Animals don't get stuck endlessly in the animal realm. They can easily sink lower and get into more extreme states of suffering. Once their bad karma has been exhausted, a past good Karma may come forward to give them a better life.
There is a classification of Karma, according to the priority of effect:
Katatta Kamma are usually the Kamma done in past lives. So if #1,#2,#3 are missing, Katatta Kamma will get a chance to result in the next birth. But that is highly unlikely as animals mostly have habitual and proximate Kammas. So the chances are that they will keep sinking lower. But once you get into the Niraya, you probably won't be creating any new significant Kamma. Because you will be in a constant state of agony throughout the lifespan.
Upvote:1
Buddha said that you can transfer merits to hungry ghosts or give them food/clothes by burning it to then, a type of offering. as Kulatha said, you cannot transfee merits to animals
Just to add: There is a famous story when the Buddha said: Imagine a turtle living inside the ocean, in this vast ocean there is a small piece of wood floating, this turtle comes to the surface to brieth once in every 100 years, the chance of this turtle hits the wood when coming to the surface is higher than getting a human rebirth if you are in the low planes, animals cannot keep moral precepts, meditate or make donations, therefore they shall only return when they clear almost all negative kamma as Kulatha pointed out
Upvote:1
Just like Wiki mentioned; Karma means action, work or deed. You actually have to take good action to achieve good Karma. Basically, you cannot transfer Karma. Even Preta need to express sadhu (appreciation) to get good Karma after someone do good Karma and share it for them.
However, you can provide good environment for them (can be human/animal) to do good Karma. For example; some children/animal, they do not really understand what are Dhamma talks/chanting. If they listen and feel good somehow in it, at least they gain some good Karma.