When an escaped and injured circus wolf is found in the bush, a selfish boy denied owning a dog nurses the wolf in his wardrobe hiding it from his mother’s prying eyes, a determined dog catcher and the cruel circus owner who wants to give the wolf to an animal testing laboratory.

Either works. Well done. Love it!

What about 'When a selfish boy finds an injured wolf, he must hide if from its cruel owner or lose it to a lab'?

You have too many antagonist in the original logline, that doesn't mean they shouldn't be in the script, it's just making the logline a little long.

Here is a logline attempt with only one antagonist:
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"When he saves an injured wolf from a cruel circus owner, a selfish boy must hide the creature or lose the lupine to the lab to which his former owner has sold the beast."
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Hope that helped, good luck with this!