In Uddhava Gita of 11th Canto of Srimad Bhagavatam, Lord Krishna says the following verse
SB 11.20.17 — The human body, which can award all benefit in
life, is automatically obtained by the laws of nature, although it is
a very rare achievement. This human body can be compared to a
perfectly constructed boat having the spiritual master as the captain
and the instructions of the Personality of Godhead as favorable winds
impelling it on its course. Considering all these advantages, a human
being who does not utilize his human life to cross the ocean of
material existence must be considered the killer of his own soul.
Also this chapter of 7th Canto of Srimad Bhagavatam describes importance of human form of life.
No doubt human life is most suitable and since animals are covered by mode of ignorance, it is difficult. But even animals and humans almost living animal lufe can get liberated in certain circumstances.
SB 2.7.46 te vai vidanty atitaranti ca deva-māyāṁ
strī-śūdra-hūṇa-śabarā api pāpa-jīvāḥ yady
adbhuta-krama-parāyaṇa-śīla-śikṣās tiryag-janā api kim u
Women, sudras, Hunas, Sabaras and other sinfully born persons also
certainly cross over the material world and know the Lord if they
learn about the qualities of the devotees of the Lord. If even
animals can understand, how much more quickly can humans, who hear
from the mouth of guru, understand the Lord?
SB 2.4.18 — Kirāta, Hūṇa, Āndhra, Pulinda, Pulkaśa, Ābhīra, Śumbha,
Yavana, members of the Khasa races and even others addicted to sinful
acts can be purified by taking shelter of the devotees of the Lord,
due to His being the supreme power. I beg to offer my respectful
obeisances unto Him.
Even gods in heaven aspire for human form of life
SB 5.19.21: Since the human form of life is the sublime position for spiritual realization, all the demigods in heaven speak in this way: How wonderful it is for these human beings to have been born in the land of Bharata-varsa. They must have executed pious acts of austerity in the past, or the Supreme Personality of Godhead Himself must have been pleased with them. Otherwise, how could they engage in devotional service in so many ways? We demigods can only aspire to achieve human births in Bharata-varsa to execute devotional service, but these human beings are already engaged there.