Altruism in horses - Shetland pony defends little foal against stallion

Little Tibor (Shetland) helps his little friend (1y) against the 'danger'.
The stallion is a crossbreed Lusitano/PRE (Andalusian)

There is no family relationship in them, they know each other now about 4 month now. Still this behavior fascinates me, why does he do it because it does not benefits him (Tibor) it benefits the little one so it might fit in the definition of altruism.

Interesting to know is that Tibor (d.brown) was raised with a gelding he adored, and later a stallion that friend to rape him many times. So he is 'experienced' in the way a stallion makes advances. It might be of influence.

When the foal came to us, at the age of 4 month, Tibor did not like him at all.
https://www.youtube.com/watch?v=GOYk_T5h7RA

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