Hi Fjord Fan,
I have seen many of these Hernia's on my father's foals at his breeding farm. These hernias seem to vary in their severity and degre. In people there a many mnay types of hernias we can have. In horses I think there are something like 3 differnet types: Inguinal, scrotal and Umbilical.
This picture does show a lump there and if it were my foal I would have the vet com and check to see what is really going on and what can be done about it.
I have seen much bigger ones than this and can remember the foals having to have special care done for them in dealing with the issues.
Have you had your vet out? If not and you can do so I might start by asking if you might could send some pictures via e-mail and then ask if they can come and take a look. Some of thee hernias can be easily addressed and put back into the foal, but other times it can get bigger and require surgery to get the contents back inside the abdomen.
At this time from your photo's it appears to be small, but I do not think that excludes it from getting larger and more comples or serious of an issue.
Somtimes an umbilical hernia can include some of the intestines.
I think the umbilical ones can happen when the young little foals are playing and rearing and bucking ...that what foals do when they play. I am totally guessing on this but I wonder if some of them are like heareditary because I think I can remember certain mares of my dads...dropping foal that alost always seem to get the hernias, so I thik some may be more suseptibal than others..
If I were you I would have the vet out to check or send pics via e-mail and see what they have to say. As far as how the treat ones that on a smaller scale I am not sure what they do...
Hope this helped a bit!