Singular: et dyr - dyret (=an animal - the animal) Plural: dyr - (de) dyrene (=animals - (those) animals)
You will have to look to the context, or to other words in the sentence indicating singular or plural when "dyr" is used. For example:
"Det er et lite dyr" =(That is a small animal) In this sentence, "et" will define "dyr" as singular.
"Det er noen dyr der borte" (=There are some animals over there) Here, "noen" will indicate plural meaning of "dyr".
It seems that there is a lot of confusion in regard to the difference in the proper use of "å bo" and "å leve". Although those verbs can each, for instance, be properly translated into English as "to live", they are not interchangeable in Norwegian.
"Å bo" is properly used only in respect to one's place of residence. Examples: "Hvor bor du?" "Jeg bor i Storgata 7." "Han bor i Kina." osv.
"Å leve" is used only in respect to the conditions under which one lives. Examples: "Han lever et godt liv." "De lever i fattigdom." "Dere lever i fred." osv.
However, Norwegians, like other peoples, commonly use expressions in ways that are exceptions to the rules. This may be one such incident.
If the intention in the example is to state that some animals make their homes underground, then "Noen dyr bor under bakken," would be a grammatically better sentence, but not necessarily a more commonly used sentence.
If the intention is to inform readers about the conditions (possibly considered poorer than those above ground), then only the use of "lever" would be correct.
Noen CAN be used in singular too, yes! But not in this case, because of the inflection of "noen", which implies that it is either a feminine/masculine noun or plural. Dyr is neuter, so it has to be plural. Here's an example sentence in singular "Det er noe dyr i huset mitt" = "There's some animal in my house".