There is no singular vs plural distinction in present-tense verbs in Gaelic, tha is just is, am, are, etc. depending on the subject, here the subject is four pigs, so tha ceithir mucan… means literally four pigs are….
The agad part means at you (sg.), at thee, by you’re side, at your place; the singular 2nd person form of the preposition aig at, by. That’s how Gaelic expresses the idea of having something – something is at you.
You should read the tips and notes to the lessons (they are available in the web-browser version of Duolingo, they aren’t in the mobile app). It is explained in the tips of the Pets skill. You can also read them on the duome.eu website.