Agree with Learnin, that it is probably a leech.

Need to take a salt shaker with you to the pond. I do NOT recommend pulling the leeches off of little kids. Once they see blood, they have a chance of being scared to swim in your pond. Just shake some salt directly on the leeches and they will start to "melt" and should fall off fairly quickly.

If kids have been wading on your beach, make sure to check between their toes at the end of swimming time. The leeches can latch in there and hide and survive for quite a while.

I suspect your smaller fish actually love to snack on leeches. Hopefully, they will cull most of the leeches from your pond over the next several weeks.