What's the meaning of "Seasoned as old leather"?

Question

I checked Longman Dictionary and there are three definition for "Seasoned":

1 [only before noun] used to describe someone who has a lot of experience of a particular thing

2 seasoned food has salt, pepper etc added to it

3 seasoned wood has been prepared for use by drying

However I do not understand meaning of the phrase.

Could you please tell me what the meaning of "Seasoned as old leather" is?

The full text is here:

When Grandpa-down-the-hill was a young man, there’d been herds of livestock spread across the mountain, and they were tended on horseback. Grandpa’s ranching horses were the stuff of legend. Seasoned as old leather, they moved their burly bodies delicately, as if guided by the rider’s thoughts.

Answer 1

Longmans meaning (1). Also Cambridge:

having a lot of experience of doing something and therefore knowing how to do it well:

a seasoned traveller
a seasoned campaigner for human rights

The horses are seasoned, that is they are very experienced in their role, in this case carrying humans who are tending cattle. Also, a simile is being used - leather, e.g. that of a boot or jacket which has been worn and used over a long period, becomes 'seasoned' - soft and supple and well adapted to the wearer.