Index seek much slower with OR condition compared with separate SELECTs
For the first two queries all it has to do is scan in the clustered index to the first entry for that value of IDUkazatel - because of the order of the index that row will be the lowest value for cas for that value of IDUkazatel.

In the second query this optimisation is not value and it is probably seeking to the first row for IDUkazatel=24 then scanning down the index until the last row with IDUkazatel=25 to find the minimum value of cas over all those rows.

If you hover over that fat arrow you'll see it is reading many rows (certainly all those for 24, probably all those for 25 too), whereas the thin arrows in the plan output for the other two show the top action causing it to only consider one row.

You could try run each query and then get the minimum for the minimums found:

SELECT MIN(cas)
FROM   (
        SELECT cas=MIN(cas) FROM PenData p WHERE p.IDUkazatel = 24
        UNION ALL
        SELECT cas=MIN(cas) FROM PenData p WHERE p.IDUkazatel = 25
    ) AS minimums


That said, it seems you have a table with IDUkazatel values rather than an explicit OR clause. The code below will work with that arrangement, simply replace the table name @T with the name of the table containing IDUkazatel values:

SELECT 
    MinCas = MIN(CA.PartialMinimum)
FROM @T AS T
CROSS APPLY 
(
    SELECT 
        PartialMinimum = MIN(PD.Cas)
    FROM dbo.PenData AS PD
    WHERE 
        PD.IDUkazatel = T.IDUkazatel
) AS CA;


In an ideal world, the SQL Server query optimizer would perform this rewrite for you, but it does not always consider this option today.