No not enough. A .22 LR would do fine, with a GOOD hit in the head which is easy since they often swim nose up, and you can shoot a little behind the nose. A .22 should not scare the horses, especially if you make sure the horses see you are out. Thank you though for considering them.
If the folks have a Muskrat likely there is more then one, and soon to be a bunch when they have little ones. This is a major problem, find where the Muskrats have been digging,. Usually they like to dig into the dam. Soon they are all they way thru, and you find your pond has drained. That is the entire, problem, and why you really need to get rid of the muskrats.
Soon however, will be trapping season. You can then use traps at the tunnel entrance, and let them do the work. If you don't want to do that put an ad on Clist sporting section, and ask for a trapper to come trap muskrats. Bet, you get a guy smiley happy to do it for you. I believe the pelts are way down in value now, only about $5.00 each.
I would just get my lawn chair, do a little cat fishing, and keep the .22 close at hand ( No I wouldn't I prefer to make fish food. The .22-250 would be my choice. Was that moss flying, or shredded Muskrat ? ). BE VERY CAREFUL IF FIRING A .22 INTO WATER. Make certain the angle is steep to avoid a ricochet OR be sure you let the ricochet goes to a safe direction. .22 are NOTORIOUS ricochet hazards.