das Schweinchen
piglet
The diminutive form of Schwein (pig); a young or small pig, a piglet.
Auf dem Bauernhof konnten die Kinder die kleinen Schweinchen füttern.
On the farm, the children were able to feed the little piglets.

piglet / piggy bank
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piglet
The diminutive form of Schwein (pig); a young or small pig, a piglet.
Auf dem Bauernhof konnten die Kinder die kleinen Schweinchen füttern.
On the farm, the children were able to feed the little piglets.

The word 'Schweinchen' (little pig / piglet) is the diminutive form of 'das Schwein' (the pig). The ending '-chen' makes a noun neuter (das) and expresses that something is small, young, or cute.
Das kleine Schweinchen läuft zu seiner Mutter.
The little piglet runs to its mother.
You can recognize the meaning of 'Schweinchen' (piglet / piggy bank / little piggy) from the context. If it is about animals, it means a small pig. If it is about money, it means a piggy bank. If it refers to a person, it is an often affectionate description of messiness.
Wirf die Münzen bitte in dein Schweinchen.
Please put the coins in your piggy bank.
A pig or piglet is considered a symbol of good luck in Germany. A 'Glücksschwein' (lucky pig), often made of marzipan or as a small charm, is a popular gift for New Year's or before exams.
Sie hat mir als Glücksbringer ein kleines Schweinchen aus Porzellan geschenkt.
She gave me a little porcelain pig as a lucky charm.
In connection with a piggy bank, figurative verbs are often used. You 'feed' the piggy bank with money, and you 'slaughter' it when you take the money out.
Ich muss mein Schweinchen schlachten, weil ich Geld für den Urlaub brauche.
I have to break open my piggy bank because I need money for the vacation.
When a person is called a 'Schweinchen' (little piggy), it is not always negative. With children, the expression is often used affectionately to say that they have gotten dirty.
Nach dem Essen im Garten war mein Sohn ein glückliches, kleines Schweinchen.
After eating in the garden, my son was a happy little piggy.
sow / pig
'Sau' and 'Schweinchen' can both have to do with pigs or with a messy person. As an animal word, 'Sau' is a female domestic pig, while 'Schweinchen' means a small or young pig. As a word for a person, 'Sau' is crude and insulting. 'Schweinchen' sounds much softer and can also be meant jokingly or affectionately, for example with a child who has got dirty. The words can be confused because both come from 'Schwein' (pig). But you should be careful: 'Sau' can sound very rude, while 'Schweinchen' is usually much more harmless.

Nach dem Spielen im Garten sieht das Kind aus wie ein kleines Schweinchen.
After playing in the garden, the child looks like a little piggy.
Auf dem Bauernhof füttert der Bauer die Sau und ihre Ferkel.
On the farm, the farmer feeds the sow and her piglets.