(And others of its kind...)

(And others) — meaning, and another [thing] to be tasted. Ibn Mas‘ud interpreted it, as narrated from him by a group, as al-zamharir (intense cold), or another [type of] punishment.

Al-Hasan, Mujahid, al-Jahdari, Ibn Jubayr, ‘Isa, and Abu ‘Amr read it as ukhar (plural), meaning other things to be tasted or other types of punishment.

(Of its kind) — meaning, of the like of this [thing] to be tasted or this punishment in terms of intensity and horror. The pronoun is singular rather than dual, considering the hamim (boiling water) and ghassaq (pus/foul discharge) as being mentioned, or referring to the "drink" which encompasses both hamim and ghassaq, or referring specifically to ghassaq. Mujahid read shiklihi with a kasra on the shin, which is a dialectal variation, just as in mithl (likeness); if it meant "coquetry," it would be with a kasra exclusively.

(Pairs/Types) — meaning, species.

On both readings, akhar (or ukhar) potentially functions as the predicate of an omitted subject; that is, "This is another thing to be tasted or another punishment," or "These are other things to be tasted or other types of punishment." The sentence is coordinated with "This is hamim." If you wish, estimate the subject as "it" and coordinate the sentence with hamim. It may also be an initial subject whose predicate is omitted; that is, "And from it is another thing to be tasted or another punishment," or "And from it are other things to be tasted or other types of punishment," with coordination to "[And from it is] hamim." It has also been permitted to estimate the predicate as "[is reserved] for them," that is, "And for them is another thing to be tasted or another punishment," or "And for them are other things to be tasted or other types of punishment," with coordination to "[This, let them taste it]."

In all these instances, "of its kind" and "pairs" are adjectives for akhar or ukhar. Although akhar is singular in wording, it is plural in meaning and applies to multiple things.

It is also possible that akhar is the subject, "of its kind" is its adjective, and "pairs" is the predicate. The answer regarding the lack of agreement in the singular reading is what you have heard. [Another view is] that it is coordinated with hamim (coordination to the singular), and "of its kind" is its adjective, while "pairs" is an adjective for the three coordinated items. It has been permitted that akhar is the subject, "of its kind" is its predicate, and "pairs" is the agent of the prepositional phrase. It has also been permitted that the first [akhar] is the subject, "of its kind" is a fronted predicate, and "pairs" is an initial subject, with the sentence functioning as the predicate of the first subject (i.e., akhar). Beginning the sentence with it is valid, for it falls under the category of "The weak took refuge in a tamarisk shrub," meaning the subject is in reality an omitted noun being described: "another type" or "another thing to be tasted." It is said that this is because it was introduced for the sake of detailing. Among the justifications they mentioned is that an indefinite noun may serve as a subject if it is for the purpose of detailing, such as: "People are two men: a man I honored, and a man I humiliated." Ibn Hisham examined this in al-Mughni.

They made the pronoun in "its kind," according to both views, refer back to akhar. These two views can hardly be reconciled with the reading of the plural. So, contemplate this, and do not be heedless.