Merhaba.

I found this sentence as an example how to practically use the verb "eşelenmek":

"İleride, ilkbahar keyfi içinde, dört ayağı havada eşelenen boz merkebi görüyor musunuz?"

But I have almost no clue what can mean...

"Do you see the gray donkey which is digging through four legs in the air later on"?

That´s a total non sense..

The person is describing a donkey which is  further up.

 Can you see  [there is] a gray donkey further up  in a spring mood [enjoying the spring] having his four legs up in the air scratching about [digging about] ?

"İleride, ilkbahar keyfi içinde, dört ayağı havada eşelenen boz merkebi görüyor musunuz?"

Ohh, thanks.

But actually, from this description I can´t imagine what the donkey was really doing

If he has 4 legs in the air, so what is he standing on??

Obviously he is not standing, He is laying on his back with his legs pointing the air playing He is joyful and  just enjoying the spring.