Bonded Horncrest and Prey Upon interaction
If Prey Upon is used on Bonded Horncrest, will it be able to "attack alone"?


No, it won't. Prey Upon is a spell that makes creatures fight each other. Fight is a keyword:


  701.12. Fight
  
  
    701.12a A spell or ability may instruct a creature to fight another creature or it may instruct two creatures to fight each other. Each of those creatures deals damage equal to its power to the other creature.
  


I assume you got confused by the wording on the cards, but fighting has absolutely nothing to do with the combat phase. If a creature (such as Bonded Horncrest) cannot attack alone, it means that it cannot be the only creature declared as an attacker in the "declare attackers" step of the combat phase.


  p.s. If Prey Upon is used on Snapping Sailback, will it get a +1/+1 counter?


Assuming Snapping Sailback survives the damage it was dealt, then yes, it gets a +1/+1 counter.


  and I assume the +1/+1 counter enters after the fight/dmg has been dealt, right?


That's correct.