Sounds like a chimpanzee.
small, humanoid
Obviously, that's what they are.
clothes
Anything can wear clothes. The hosts expect it of them and train them as such. Or, they may need warmth in the climate same as the humans.
and a beard.
Ok, they don't shave. Again, nothing odd here except for style choices.
They live on farms and in houses where they help out with farmwork and other labour, though they stay out of sight. They don't require much except being respected and having food (like porridge) offered to them once in a while.
That describes any smart domesticated or socialized animal. You could be talking about your dog here, or the barn cats.
Their abilities include having the strength of several grown men
This in particular makes me think of nonhuman apes. A chimp has enormous strength relative to us.
If offended somehow, their behaviour switches around to vengeance and spite.
Nothing novel here. Non-human intelligence like a farm animal or pet does that.
despite their small size and being good at taking care of animals.
They learn the chores and can perform them. Dogs learn their roles but it sounds like you mean general farm hands which require hands. Have you heard heard about the baboon who got a job as a railroad switch operator?
Most of what you describe is not special. What is specific sounds like it could be a real-world ape. More interesting to the story would be to make them a homonid that did not go extinct but took up with the modern humans when they started agriculture.
Nothing special is needed to explain their form and abilities. All that is normal for a homonid and for most of their existance there have in fact been multiple hominin species extant at the same time.
So, consider the story of wolves and modern humans and contrast with the case of neandertals. Suppose that a species of hominin smaller than moderns were hunter-gatherers. Accept the general idea that moderns traded muscle strength for finesse: being able to throw things accuratly became selected for and we lost the raw strength of our relatives. Just as some wolves decided to throw their lot in with the modern humans, these hominins did something similar. They contributed their greater strength and efficency (does chores for less food than a grown modern human) and got the benifits of sharing agriculture, clothing, expansion into new areas, etc.
Just as dogs evolved breeds based on their roles, these tomte evolved to fit their role, especially if the modern humans influence their breeding.
So they have the specific traits they were adapted for:
They have great raw strength. They can lift and carry, but can't throw worth anything. They can’t thread a needle or make precision tools.
They are smart enough to do their work, but have a small brain and need half the calories of a modern human at rest. The brain uses a lot of power. That along with smaller overall size saves energy.
Living with us and eating the same food, they also lost the huge chewing muscles and long gut, and rely on cooking of cultivated food. But like dogs they eat what we throw away. They scavenge most of their diet and consume byproducts generated as part of their work so we don’t generally notice that. But they need some dietary suppliments such as cooked grains and some fruit. So, the farmer prepares them porridge, sharing his own breakfast, but they eat on their own throughout the day.
