Learn French: Nous avons recueilli un chien errant hier.

While recueillir can mean 'to gather' or 'to collect' (like data or rainwater), when used with people or animals, it specifically means 'to take in' or 'to give shelter to'. It implies an act of rescue or providing a safe haven, which makes it the standard verb for taking in a stray animal.

errant looks like an -ing verb. Is it functioning as an adjective here?

Yes. errant is the present participle of the verb errer (to wander or roam), but here it acts entirely as an adjective meaning 'stray'. Because it is functioning as an adjective, it must agree with the noun it modifies. If you were talking about a female dog, it would be une chienne errante.

Why is hier placed at the very end of the sentence instead of between the auxiliary verb and the past participle?

Adverbs of specific time, like hier (yesterday), aujourd'hui (today), or demain (tomorrow), usually go at the very beginning or the very end of a sentence. Unlike short adverbs (such as souvent or déjà), they are almost never placed between the auxiliary verb (avons) and the past participle (recueilli).

The past participle recueilli looks intimidating. What verb family does it belong to?