>1 single pen Observing more than one animal in a single pen can be approached in a number of different ways, depending on what you are trying to test/observe and how many animals there are in the pen. If you are comparing treatments (presence of enrichments, for example) on behavioural response x, typical designs include (but are not limited to) phase designs such as:

AB
ABA
ABAB

where 'A' is baseline and 'B' is treatment x.

These types of design are very useful as the allow you to return to baseline to ensure that any effects of treatment are not due to uncontrolled factors.

Finally, don't forget that as you have not used random sampling (i.e., all of the animals have come from one pen) you cannot generalise your findings beyond this study! To do this, you would need to collect a random sample from the population.

Some other considerations:

If you require individual identification, take care when choosing marking substrates as some may interfere with behaviour. If possible, identify individuals by natural markings. In many farms, animals are marked as a husbandry procedure, so use this to your advantage.

If possible, include enrichment devices in pens, as overly standardized environments can adversely affect the outcome of experiments (unless this is part of the question you are asking!).

It is a good idea to plan to carry out at least one (preferably more than one) independent replication of the study.