"You have a frog."
Translation:Tha losgann agad.
No, neither proper nor acceptable - it just doesn't make sense. In Gaelic, sentences that mean "x has y" take the form tha y aig x, literally "is y at x" (or a bit less literally, "y is at x"), i.e. the object of the English sentence is the subject of the Gaelic sentence.
aig changes form when combined with a pronoun - aig +mi = agam, aig + thu = agad. So tha losgann agad complies with the formula tha y aig x - "is a frog at you"/"a frog is at you" = "you have a frog". The thu you are looking for is hiding inside agad, but it's there nonetheless.
Tha thu losgann agad has two subjects, one of which is also the object, and doesn't fit the formula - here we have tha x y aig x - and therefore doesn't make sense.
Sentences in Gaelic often have quite a different perspective from sentences in English - e.g. the English object is often the Gaelic subject, and prepositional pronouns (the combination of a preposition and a pronoun - agam, agad, orm, ort, etc.) are a vital part of Gaelic grammar, so it's worth spending time getting your head round them now - the Tips at Pets introduces the agam structure (also in the full Duome tips: https://duome.eu/tips/en/gd)