Because of Buddha's great compassion, he can spontaneously manifest in many different forms in order to guide and benefit sentient beings. Sometimes they emanate as high level bodhisattvas , and the form of such a bodhisattva counters specific purpose or in a way that one would never even expect that a Buddha may manifest.. ways that are against our beliefs and understanding.
He appears in different forms according to a being’s karma, and that Buddhas appear in skilful ways. They don’t announce themselves. But they can act in very subtle ways to influence other people, so that those people begin to create good karma, they begin to get an idea of ethics, they begin to practise bodhicitta and so on. They say that a Buddha can appear as one of our friends, as a dog or as a cat, or in any other form, so long as they can help us. Once again, these aren’t announced and they often come and go, so we don’t even recognize them.
That is why we are always taught to treat every being the same be they thieves, criminals or animals. Hence, they are our means of practice.
A story to share on Asanga Meeting Buddha Maitreya
Asanga was one of the famous buddhist saints that was born during 300 C.E. and lived for seventy years, he was also the founder chittamatra or Mind-Only School, one of the branches of Madhyamika.
At one period of time, he went to the Gurpa Mountain for a long solitary retreat on uddha Maitreya. He was hoping to get some accomplishments in the practice and meet Buddha Maiterya.
After 3 years of meditation(3rd year), he was getting frustrated because there were still no signs of any improvement or even an auspicious dream. So he withdraw from his retreat and left the mountain.
On his way, he saw some birds sitting on a big rock. There was a thin line of crack on the rock beside these birds. He thought to himself that it must be the wings of the birds for the past few years that have brush on the same spot that causes the crack. This struck upon him that with perseverance, he will succeed in the meditation too. So he went back to the cave to meditate on Buddha Maitreya again.
Then another 3 years of retreat went pass(6th Year), he was not making any progress, again feeling dishearten, he decided to leave the cave. On the foot of the mountain, he saw a man polishing an iron bar with cloth.
Asanga asked him what was he doing, the man replied that he is going to make a Needle out of this iron bar. Asanga thought to himself that this man is totally ridiculous, however admiring his determination, Asanga was inspire that meditation will still be easier rather than making a needle out of an iron, and so he went back to the cave and meditate on Buddha Maitreya.
So another 3 years passed (9th year), still with no success, so he left the retreat again and this time he met another man who was brushing a soaked feather against the rock. Asanga ask him what was
he doing, the man reply that he is trying to corrode this big rock because it is preventing the sun to shine into his house. Feeling ashamed of his lack of determination and commitment, he thinks that anyway meditating on Buddha Matreiya will be much more easier. So he went back to his cave and meditate again.
Another 3 three years passed (12th year), still making no progress, feeling miserable and despair, he felt that due to his bad karma and obscuration, he decided to withdraw after his 12 year retreat. So he left the cave.
On the way, he found an injured dog with the lower part of the body covered with maggots. Deep compassion arose within him for both the dog and the maggots, in order not to kill the maggots, he close his eyes and use his tongue to lick up the maggots, immediately his tongue was touching the ground before he knew.
He open his eyes to see what happen and saw Buddha Maitreya right in front of him. Astonished, Asanga said, "For so many years, I have been meditating on you, why until do you appear before me ?"
Buddha Maitreya gently replied that he has always been beside him from the beginning,it was due to his many obscuration going around his mind and compassion that obstruct Asanga from seeing him. When Asanga genuine compassion arises for the dog, all these negativity are cleared.
Buddha Maitreya asked Asanga to carry him on his back to the city, Asanga asked around the people in the city what is he carrying, everyone replied saying there is nothing behind him. However a old lady responded that she saw Asanga carrying a injured Dog behind his back.
Therefore Buddha Maitreya explained that this old lady had recited a lot of 6 syllabus Mantra in her life(Mani Mantra), much of her obscuration are cleared. However there is still some lacking in her genuine compassion, that's why she cannot see my true form.