Does a land with the Mountain subtype tap for red mana even if it has no printed tap ability?

Under CR 305.6, any land with a basic land type — Plains, Island, Swamp, Mountain, or Forest — gains a corresponding mana ability intrinsically. This ability is not printed on the card; it is granted by the rules themselves. For Mountain, that ability is '{T}: Add {R}'.

This means that if a non-basic land somehow gains the Mountain subtype (for example, via an effect like Blood Moon, which reads 'Nonbasic lands are Mountains'), it immediately gains the ability to tap for red mana, even if it previously had no tap ability or had completely different abilities.

The key rules at work are CR 305.6 (basic land types grant mana abilities) and CR 205.4b (listing the five basic land types). The land does not need to be a 'basic' land rarity-wise — only needing the subtype Mountain is sufficient.

Concrete example: Suppose your opponent controls a Breeding Pool (an Island Forest). If Blood Moon enters the battlefield, Breeding Pool loses its island and forest subtypes and becomes just a Mountain. It now taps only for {R}, even though 'tap for red mana' was never printed on the card.

Note that the gained mana ability from CR 305.6 can be overridden or lost if the land later loses the Mountain subtype, since the ability is tied directly to having that subtype at any given moment.