I don't have pictures but what kind of semi-aquatic mammal uses slides on the bank of a pond but does not cut down trees or build huts. Mink? The slides are fairly small.

Otter maybe??

Rats

Could it be otter?

Otter is a pretty good idea. My brother caught one while beaver trapping in this pond. But it isn't beaver, 100% sure.

Since you said the slides are fairly small my guess would be muskrats.

Pig castrator...I'd go with otter

Muskrats will polish a crossover pretty slick. They don't travel very far from home so use them a lot.

If so, probably otter.

Rats

Update; So I went scouting again on this pond and there is a river 30-40 yards away and I found a crossover. But the problem is that on the pond side, the water is 2-3 inches deep where the slide enters the water. Shallow water goes 10 feet out at least. How do you trap this. I still don't know if it is muskrat or otter.

This crossover is very used and enters the river where there is a water runoff/ditch/shore.

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Just set it stick out of the water and putting guide sticks?

rats and otter and beaver and mink often use the same crossovers.Rats often have a toilet on the main crossover they use.
Rats and mink can have several crossovers along the same high ground in close proximity to each other.
Otter can have several crossovers also.
Usually there is a main one and one or two others that can be not so noticeable.

A rat slide should be 5 or 6" wide and a beaver or otter 10-12" wide

I trapped a pond once that had another pond next to it.
Found two trails in the grass that the muskrats were using.
Looking at them close , one was for going into the other pond , while the other was for coming back from it.
Muskrats are easy to snare , as the stay right in the center of their trails.
Two fingers up , 2 1/2 inch to 2 3/4 opening dead center in the path. Loaded snare is all you need.
Watch how they move in the snow , it'll prove my point. That is if they're all going and coming from one place to another.
If they're feeding it depends on how much of a food source there is.

Do otters/muskrats live in dens in trees? Because there is a tree with a trail from it going to the water.

There may be an under water entrance to the root system at that tree that comes up there.
No they don't den in trees.

Just to clarify there is a hole at the bottom of the tree but you probably realize that.