Is it only humans who can understand Buddhism?

There is a series of suttas at the very end of the Samyutta Nikaya, namely, SN 56.114 to SN 56.131, which appear to state 'hell beings', 'animals' & 'ghosts' can realise the Four Noble Truths.

I'm assuming that by "understand Buddhism" you mean achieving arahantship.

The Buddha is known as the "teacher of humans and devas" so at least both humans and devas.

In many suttas The Buddha gives teachings and instructions to devas.

Devas seem to be just like humans basically like extraterrestials, except most are longer-lived, experience more pleasure, are better looking with better brains, better senses, etc...many are possessed of iddhi.

It is also likely that others like nagas, asuras, yakkhas, and other beings intelligent enough would be able to understand Buddhism because Maha-samaya Sutta (DN 20) mentions some of them as having iddhi power and visiting and taking refuge in The Buddha.

Nagas seem to be humanoid reptile-like beings.

But in general most humans (and other beings in the universe) won't be able to understand Buddhism well enough to achieve arahantship and some are actually incapable.

"Monks, one who hasn't abandoned nine things is incapable of realizing arahantship. Which nine? Passion, aversion, delusion, anger, resentment, arrogance, insolence, envy, & stinginess. One who hasn't abandoned these nine things is incapable of realizing arahantship." (AN 9.62)

You could say that only arahants, paccekabuddhas, and sammasambuddhas understand Buddhism.

Also even beings that are willing and capable of achieving arahantship still won't, The Buddha uses similes of attempting to make a fire with wet wood:

"Then these three similes — spontaneous, never before heard — appeared to me. Suppose there were a wet, sappy piece of timber lying in the water, and a man were to come along with an upper fire-stick, thinking, 'I'll light a fire. I'll produce heat.' Now what do you think? Would he be able to light a fire and produce heat by rubbing the upper fire-stick in the wet, sappy timber lying in the water?"

"No, Master Gotama. Why is that? Because the timber is wet & sappy, and besides it is lying in the water. Eventually the man would reap only his share of weariness & disappointment."

"So it is with any brahman or contemplative who does not live withdrawn from sensuality in body & mind, and whose desire, infatuation, urge, thirst, & fever for sensuality is not relinquished & stilled within him: Whether or not he feels painful, racking, piercing feelings due to his striving [for Awakening], he is incapable of knowledge, vision, & unexcelled self-awakening. This was the first simile — spontaneous, never before heard — that appeared to me.

"Then a second simile — spontaneous, never before heard — appeared to me. Suppose there were a wet, sappy piece of timber lying on land far from water, and a man were to come along with an upper fire-stick, thinking, 'I'll light a fire. I'll produce heat.' Now what do you think? Would he be able to light a fire and produce heat by rubbing the upper fire-stick in the wet, sappy timber lying on land far from water?"

"No, Master Gotama. Why is that? Because the timber is wet & sappy, even though it is lying on land far from water. Eventually the man would reap only his share of weariness & disappointment."

"So it is with any brahman or contemplative who lives withdrawn from sensuality in body only, but whose desire, infatuation, urge, thirst, & fever for sensuality is not relinquished & stilled within him: Whether or not he feels painful, racking, piercing feelings due to his striving, he is incapable of knowledge, vision, & unexcelled self-awakening. This was the second simile — spontaneous, never before heard — that appeared to me.

"Then a third simile — spontaneous, never before heard — appeared to me. Suppose there were a dry, sapless piece of timber lying on land far from water, and a man were to come along with an upper fire-stick, thinking, 'I'll light a fire. I'll produce heat.' Now what do you think? Would he be able to light a fire and produce heat by rubbing the upper fire-stick in the dry, sapless timber lying on land?"

"Yes, Master Gotama. Why is that? Because the timber is dry & sapless, and besides it is lying on land far from water."

"So it is with any brahman or contemplative who lives withdrawn from sensuality in body & mind, and whose desire, infatuation, urge, thirst, & fever for sensuality is relinquished & stilled within him: Whether or not he feels painful, racking, piercing feelings due to his striving, he is capable of knowledge, vision, & unexcelled self-awakening. This was the third simile — spontaneous, never before heard — that appeared to me." (Maha-Saccaka Sutta, MN 36)

No matter how hard someone tries to make a fire even if they are willing and capable if their method is wrong they won't succeed.

In the same way no matter how hard someone tries to achieve arahantship even if they are willing and capable if their method is wrong they won't succeed.

It is doubtful that there any arahants or paccekabuddhas in the world right now. Even during The Buddha's time with his assistance and teaching there were only a few thousand arahants in the world.

Since you could say that only arahants, paccekabuddhas, and sammasambuddhas understand Buddhism you could say that it is doubtful that anyone in the world right now understands Buddhism.

There probably are however some humans in this world who have achieved higher states (jhanas) and developed iddhi powers (concentration) but still haven't achieved arahantship (the ending of mental fermentations), and also many who mistakenly believe themselves to be enlightened.

Only humans and the beings in the higher realms can attain enlightenment. Except for the ones in certain Brahma realms. Some beings in the lower realms may hear the Dhamma, memorise it and be born in higher realms as a result.

Yes, you need to be human.

The Buddha even made the famous simile of the turtle in the ocean that comes up for air once every century and, when it does, happens to put its neck through a yoke floating around in the same ocean. He said that that's how lucky you are for having been born a human, so you shouldn't waste this opportunity to work towards enlightenment.

If you're another kind of sentient being, you'd better hope that you're reborn as a human in your next incarnation.

Kôdô Sawaki said:

"All creatures are Buddha already, but it is the human alone which needs to express his Buddhaship every day".

I don't know if this originate in a teaching of Gautama Buddha or if it is Sawakis interpretation alone. However I understand it like all humans are Buddha by default, as all other animals. But humans might "lose their way" and so its their practice alone to return to the Buddhaship.

That for me also means that animals do not need to understand Buddhism, because they are already Buddha. Understanding also would mean to try thinking about it. I doubt animals do that, and if they could, I doubt they see any benefit of trying to understand anything which is "just there".

We humans try to understand Buddhaship, as we can loose this state of mind. Bodhidharma once said, we do no need to understand Buddhism, because we would never understand as long as we want. Instead we just need to practice Buddhism, which will make you understand.

(I am practicing Zen, thats why my response might be from a view from the Zen angle)

Perhaps the question needs to be clearer. Are you talking of Buddhism as in the teachings of the Buddha or Buddhism as taught by the two main branches (Mahayana and Theravada) which could be said to be the interpretation of the teachings of the Buddha, often blended with cultural influence?

I have a pet corn snake and she could be considered to be in a mindful state for much of the time - she neither cares about the past nor fears for the future. She hunts when she's hungry and rests when there is no need for her to move. She never appears to be dissatisfied, she has no attachment to physical things and she does not crave anything beyond her basic needs for survival. By that definition, has she not already achieved Nirvana?

The challenge to human enlightenment is that we do crave, we do form attachments to experiences and possessions and we constantly experience dissatisfaction with our life experience. Can an animal that exemplifies the four noble truths be said to understand Buddhism? Not really - they are as they are - they have no need for Buddhism nor an understanding of it.

Maybe only a Buddhist can understand Buddhism. But enlightenment, do you think that an Asian have more chances to reach enlightenment than a European?

Anything based in the impermanence of our senses and mind will produce more ignorance.

This is also ignorant but I prefer lo live in a world where the enlightenment it is reachable from any living or not living form, and right here, right now.