The dog card says to pick 1 card from the deck and one card for each sheep visible. What sheep is it referring too? The sheep in my collection or the sheep in the collections of other players as well?
All visible means all the ones you can see.
The designer, Christian Giove, confirms on this BGG thread:
you count sheeps on top of ALL COLLECTIONS (your own and those of other player) plus those in the DRAW LINE and also the card on the top of the DISCARD PILE.