1. absence of "the" in Russian invites (sometimes) the usage of other determiners or change of the wording. In other words: often "the X" or "a X" corresponds to "X'.
But sometimes (sometimes) 'a X' is "один Х", "какой-то Х" etc., while 'the X' can be 'Этот Х' 'он' etc.
Think of the context, it may be the case....
2.the right translation is: .......какую только можно (было) найти в лесу, I believe:)
- было for past (obviously)
- бы is not needed, because this 'could' looks much like past of 'can' and not much like introducing some 'irrealis mood' or as in 'could you please...':) Anyway, 'можно' itself refers to a possibility. With 'бы'
- только - somehow, самая [....], какую можно [найти, увидеть...] more often than not comes with только. May be to distinguish from такая, какую можно увидеть в [Британском музее]
- the most important point. Russian has an 'adjective' row 'какой? ,какой... такой' (такой=such) along with 'как? так" (how? ) , куда? туда, кому? тому etc.
Because of the presence of this option (note, that 'which' points on the adjective 'beautiful') the most "correct" version of your phrase:
".... которую только можно найти в лесу."
sounds not great.
(without 'только' this one aquires additional meaning: "...of the most beautiful monkey. You can find her (tMBM) in the forest".)
Because it sounds not great, Alex rephrased it.
The second varian proposed by Helga is "the most literal grammatically correct translation possible". But Russion isn't fond of passive constructions.